diff --git a/library/Application/Console/App.php b/library/Application/Console/App.php
index 895f3101a..e3da9f7a6 100644
--- a/library/Application/Console/App.php
+++ b/library/Application/Console/App.php
@@ -40,6 +40,8 @@
 
 /**
  * Command line application for OPUS 4 management tasks.
+ *
+ * TODO get list of Commands from configuration/registration (allow modules to add commands, decentralize the code)
  */
 class Application_Console_App extends Application
 {
@@ -53,7 +55,11 @@ public function __construct()
         $this->add(new ExtractFileCommand());
         // $this->add(new Application_Console_Index_RepairCommand());
         // $this->add(new Application_Console_Index_CheckCommand());
+
         $this->add(new Application_Console_Document_DeleteCommand());
+        $this->add(new Application_Console_Document_DuplicatesCommand());
+        $this->add(new Application_Console_Document_DiffCommand());
+
         $this->add(new BibtexImportCommand());
         $this->add(new BibtexListCommand());
         $this->add(new Application_Console_Debug_DocumentXmlCommand());
diff --git a/library/Application/Console/Document/DiffCommand.php b/library/Application/Console/Document/DiffCommand.php
new file mode 100644
index 000000000..5d2233388
--- /dev/null
+++ b/library/Application/Console/Document/DiffCommand.php
@@ -0,0 +1,209 @@
+<?php
+
+/**
+ * This file is part of OPUS. The software OPUS has been originally developed
+ * at the University of Stuttgart with funding from the German Research Net,
+ * the Federal Department of Higher Education and Research and the Ministry
+ * of Science, Research and the Arts of the State of Baden-Wuerttemberg.
+ *
+ * OPUS 4 is a complete rewrite of the original OPUS software and was developed
+ * by the Stuttgart University Library, the Library Service Center
+ * Baden-Wuerttemberg, the Cooperative Library Network Berlin-Brandenburg,
+ * the Saarland University and State Library, the Saxon State Library -
+ * Dresden State and University Library, the Bielefeld University Library and
+ * the University Library of Hamburg University of Technology with funding from
+ * the German Research Foundation and the European Regional Development Fund.
+ *
+ * LICENCE
+ * OPUS is free software; you can redistribute it and/or modify it under the
+ * terms of the GNU General Public License as published by the Free Software
+ * Foundation; either version 2 of the Licence, or any later version.
+ * OPUS is distributed in the hope that it will be useful, but WITHOUT ANY
+ * WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
+ * FOR A PARTICULAR PURPOSE. See the GNU General Public License for more
+ * details. You should have received a copy of the GNU General Public License
+ * along with OPUS; if not, write to the Free Software Foundation, Inc., 51
+ * Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ *
+ * @copyright   Copyright (c) 2023, OPUS 4 development team
+ * @license     http://www.gnu.org/licenses/gpl.html General Public License
+ */
+
+use Opus\Common\Config\ConfigException;
+use Opus\Common\Document;
+use Opus\Common\Model\NotFoundException;
+use Opus\Common\Repository;
+use Symfony\Component\Console\Command\Command;
+use Symfony\Component\Console\Input\InputArgument;
+use Symfony\Component\Console\Input\InputInterface;
+use Symfony\Component\Console\Input\InputOption;
+use Symfony\Component\Console\Output\OutputInterface;
+
+/**
+ * Shows differences between two or more documents.
+ *
+ * TODO support different levels of detail
+ */
+class Application_Console_Document_DiffCommand extends Command
+{
+    public const OPTION_DOI = 'doi';
+
+    public const OPTION_SERVER_STATE = 'server-state';
+
+    public const OPTION_IGNORE_DELETED = 'ignore-deleted';
+
+    public const ARGUMENT_DOC_ID = 'DocID';
+
+    protected function configure()
+    {
+        parent::configure();
+
+        $help = <<<EOT
+Shows the differences between two or more documents.
+
+Multiple document IDs can be provided as argument. The fields of the documents
+are then compared and differences reported.
+
+If a DOI is provided, using the <fg=green>--doi</> Option, the database is
+searched for matching documents. If more than one is found, the differences
+between the documents are reported.
+
+<fg=red>NOTES:
+- Complex values like persons and patents are not shown with all their metadata.
+- For text values, like titles and abstracts, the exact differences are not 
+  highlighted. The report just shows that the values are different.    
+</>
+EOT;
+
+        $this->setName('document:diff')
+            ->setDescription('Shows differences between documents.')
+            ->setHelp($help)
+            ->addOption(
+                self::OPTION_DOI,
+                null,
+                InputOption::VALUE_REQUIRED,
+                'DOI value'
+            )
+            ->addOption(
+                self::OPTION_SERVER_STATE,
+                's',
+                InputOption::VALUE_REQUIRED,
+                'Include docs in state (DOI) - <fg=yellow>unpublished</>, <fg=yellow>published</>, <fg=yellow>inprogress</>, <fg=yellow>audited</>, <fg=yellow>restricted</>, <fg=yellow>deleted</>'
+            )
+            ->addOption(
+                self::OPTION_IGNORE_DELETED,
+                null,
+                InputOption::VALUE_NONE,
+                'Ignore deleted documents (DOI)'
+            )
+            ->addArgument(
+                self::ARGUMENT_DOC_ID,
+                InputArgument::OPTIONAL + InputArgument::IS_ARRAY,
+                'Two or more document IDs'
+            );
+    }
+
+    /**
+     * @return int
+     * @throws NotFoundException
+     */
+    protected function execute(InputInterface $input, OutputInterface $output)
+    {
+        $diff = new Application_Document_DocumentDiff();
+        $diff->setOutput($output);
+
+        $docIds = $this->getDocumentsInput($input, $output);
+
+        $docCount = count($docIds);
+
+        if ($docCount === 0) {
+            $output->writeln('No documents for comparison');
+        } elseif ($docCount === 1) {
+            $docId = $docIds[0];
+            $output->writeln("Only one document for comparison (ID = $docId)");
+        } else {
+            $diff->diff($docIds);
+        }
+
+        return 0;
+    }
+
+    /**
+     * @param InputInterface  $input
+     * @param OutputInterface $output
+     * @return int[]
+     * @throws ConfigException
+     */
+    protected function getDocumentsInput($input, $output)
+    {
+        $docIds = $input->getArgument(self::ARGUMENT_DOC_ID);
+
+        if (count($docIds) === 0) {
+            $doi = $input->getOption(self::OPTION_DOI);
+
+            if ($doi !== null) {
+                $finder = Repository::getInstance()->getDocumentFinder();
+                $finder->setIdentifierValue('doi', $doi);
+
+                $serverStates = $this->getServerStateInput($input, $output);
+
+                if ($serverStates !== null && count($serverStates) < 6) {
+                    $finder->setServerState($serverStates);
+                }
+
+                $docIds = $finder->getIds();
+
+                if (count($docIds) === 0) {
+                    $output->writeln("No documents found for DOI: $doi");
+                }
+            }
+        }
+
+        return $docIds;
+    }
+
+    /**
+     * @param InputInterface  $input
+     * @param OutputInterface $output
+     * @return string[]|null
+     */
+    protected function getServerStateInput($input, $output)
+    {
+        $validStates = $this->getValidServerStates();
+
+        if ($input->getOption(self::OPTION_IGNORE_DELETED)) {
+            unset($validStates['deleted']);
+        }
+
+        $serverStateOption = $input->getOption(self::OPTION_SERVER_STATE);
+
+        $serverStates = [];
+
+        if ($serverStateOption !== null) {
+            $values = explode(',', $serverStateOption);
+            foreach ($values as $state) {
+                if (in_array(strtolower($state), $validStates)) {
+                    $serverStates[$state] = $state;
+                } else {
+                    $output->writeln("Invalid ServerState value: $state");
+                }
+            }
+        } else {
+            $serverStates = $validStates;
+        }
+
+        return $serverStates;
+    }
+
+    /**
+     * @return string[]
+     *
+     * TODO better way of getting valid states (configurable states in the future?)
+     */
+    protected function getValidServerStates()
+    {
+        $doc   = Document::new();
+        $field = $doc->getField('ServerState');
+        return $field->getDefault();
+    }
+}
diff --git a/library/Application/Console/Document/DuplicatesCommand.php b/library/Application/Console/Document/DuplicatesCommand.php
new file mode 100644
index 000000000..ed6f64caa
--- /dev/null
+++ b/library/Application/Console/Document/DuplicatesCommand.php
@@ -0,0 +1,238 @@
+<?php
+
+/**
+ * This file is part of OPUS. The software OPUS has been originally developed
+ * at the University of Stuttgart with funding from the German Research Net,
+ * the Federal Department of Higher Education and Research and the Ministry
+ * of Science, Research and the Arts of the State of Baden-Wuerttemberg.
+ *
+ * OPUS 4 is a complete rewrite of the original OPUS software and was developed
+ * by the Stuttgart University Library, the Library Service Center
+ * Baden-Wuerttemberg, the Cooperative Library Network Berlin-Brandenburg,
+ * the Saarland University and State Library, the Saxon State Library -
+ * Dresden State and University Library, the Bielefeld University Library and
+ * the University Library of Hamburg University of Technology with funding from
+ * the German Research Foundation and the European Regional Development Fund.
+ *
+ * LICENCE
+ * OPUS is free software; you can redistribute it and/or modify it under the
+ * terms of the GNU General Public License as published by the Free Software
+ * Foundation; either version 2 of the Licence, or any later version.
+ * OPUS is distributed in the hope that it will be useful, but WITHOUT ANY
+ * WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
+ * FOR A PARTICULAR PURPOSE. See the GNU General Public License for more
+ * details. You should have received a copy of the GNU General Public License
+ * along with OPUS; if not, write to the Free Software Foundation, Inc., 51
+ * Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ *
+ * @copyright   Copyright (c) 2023, OPUS 4 development team
+ * @license     http://www.gnu.org/licenses/gpl.html General Public License
+ */
+
+use Opus\Common\Model\NotFoundException;
+use Opus\Doi\DoiManager;
+use Symfony\Component\Console\Command\Command;
+use Symfony\Component\Console\Helper\ProgressBar;
+use Symfony\Component\Console\Input\InputInterface;
+use Symfony\Component\Console\Input\InputOption;
+use Symfony\Component\Console\Input\StreamableInputInterface;
+use Symfony\Component\Console\Output\OutputInterface;
+
+/**
+ * Reports and removes or tags duplicate documents.
+ *
+ * TODO support --from
+ * TODO support --until
+ * TODO add option for base URL for links
+ */
+class Application_Console_Document_DuplicatesCommand extends Command
+{
+    public const OPTION_DOI = 'doi';
+
+    public const OPTION_DOI_FILE = 'doi-file';
+
+    public const OPTION_CSV_REPORT = 'csv-report';
+
+    public const OPTION_FROM = 'from';
+
+    public const OPTION_UNTIL = 'until';
+
+    public const OPTION_FORMAT = 'format';
+
+    public const OPTION_DRYRUN = 'dry-run';
+
+    public const OPTION_REMOVE = 'remove';
+
+    protected function configure()
+    {
+        parent::configure();
+
+        $help = <<<EOT
+Checks for duplicate documents using DOIs and generates a report. 
+
+If no DOIs are provided the database is searched for all duplicate DOIs. 
+
+If the <fg=green>--csv-report</> option is used to provide a file name, a CSV formatted
+report is written, containing links to the documents found. The links depend on the 'url'
+option being set in the configuration ('config.ini'). The columns in the CSV file are:
+
+  DOI, Doc-ID, Link, Date Created, Server State
+
+Duplicate documents can be removed automatically using the <fg=green>--remove</> option.   
+<fg=red>
+NOT SUPPORTED YET:
+- Tagging and linking of duplicate documents for review by administrator
+</>
+EOT;
+
+        $this->setName('document:duplicates')
+            ->setDescription('Removes duplicate documents by checking DOIs.')
+            ->setHelp($help)
+            ->addOption(
+                self::OPTION_DOI,
+                null,
+                InputOption::VALUE_REQUIRED,
+                'One or more DOI values (CSV)'
+            )
+            ->addOption(
+                self::OPTION_DOI_FILE,
+                null,
+                InputOption::VALUE_REQUIRED,
+                'File containing DOIs (one per line)'
+            )
+            ->addOption(
+                self::OPTION_DRYRUN,
+                null,
+                InputOption::VALUE_NONE,
+                'Check DOIs without making changes'
+            )
+            ->addOption(
+                self::OPTION_CSV_REPORT,
+                null,
+                InputOption::VALUE_REQUIRED,
+                'Output file for CSV report'
+            )
+            ->addOption(
+                self::OPTION_REMOVE,
+                null,
+                InputOption::VALUE_NONE,
+                'Automatically remove newest duplicate document if UNPUBLISHED'
+            );
+    }
+
+    /**
+     * @return int
+     * @throws NotFoundException
+     */
+    protected function execute(InputInterface $input, OutputInterface $output)
+    {
+        $finder = new Application_Document_DuplicateFinder();
+        $finder->setOutput($output);
+
+        if ($input->getOption(self::OPTION_DRYRUN)) {
+            $finder->setDryRunEnabled(true);
+        }
+
+        if ($input->getOption(self::OPTION_REMOVE)) {
+            $finder->setRemoveEnabled(true);
+        }
+
+        $doiValues = $this->getDoiInput($input, $output);
+
+        if (count($doiValues) === 0) {
+            $output->writeln('Searching for duplicate DOI values in database ...');
+            $doiValues = $this->getAllDuplicateDoiValues();
+        }
+
+        // Processing
+
+        $doiCount = count($doiValues);
+
+        if ($doiCount > 0) {
+            $csvPath = $input->getOption(self::OPTION_CSV_REPORT);
+            if ($csvPath !== null) {
+                $csvFile = fopen($csvPath, 'w');
+                $finder->setCsvFile($csvFile);
+            }
+
+            $output->writeln("Checking {$doiCount} DOI values");
+
+            $progressBar = null;
+
+            if ($output->getVerbosity() === $output::VERBOSITY_NORMAL) {
+                $progressBar = new ProgressBar($output, $doiCount);
+                $finder->setProgressBar($progressBar);
+            }
+
+            $finder->removeDuplicateDocuments($doiValues);
+
+            if ($csvPath !== null) {
+                fclose($csvFile);
+            }
+
+            if ($progressBar !== null) {
+                $progressBar->finish();
+                $output->writeln('');
+            }
+        } else {
+            $output->writeln('No DOI values found');
+        }
+
+        return 0;
+    }
+
+    /**
+     * Reads DOI values from STDIN or file.
+     *
+     * @param InputInterface  $input
+     * @param OutputInterface $output
+     * @return string[]
+     */
+    protected function getDoiInput($input, $output)
+    {
+        $doi = $input->getOption(self::OPTION_DOI);
+
+        if ($doi !== null) {
+            $doiInput = preg_replace('/,/', "\r\n", $doi);
+        } else {
+            $doiFile = $input->getOption(self::OPTION_DOI_FILE);
+
+            if ($doiFile !== null) {
+                $doiInput = file_get_contents($doiFile);
+            } else {
+                if ($input instanceof StreamableInputInterface) {
+                    $inputStream = $input->getStream();
+                }
+
+                $inputStream = $inputStream ?? STDIN;
+
+                stream_set_blocking(STDIN, 0);
+
+                $doiInput = stream_get_contents($inputStream);
+            }
+        }
+
+        if (strlen(trim($doiInput)) > 0) {
+            $doiValues       = preg_split("/((\r?\n)|(\r\n?))/", $doiInput);
+            $uniqueDoiValues = array_unique($doiValues);
+
+            if (count($doiValues) !== count($uniqueDoiValues)) {
+                $output->writeln('Duplicates entries removed from DOI list.');
+            }
+
+            return $uniqueDoiValues;
+        } else {
+            return [];
+        }
+    }
+
+    /**
+     * @return string[]
+     */
+    protected function getAllDuplicateDoiValues()
+    {
+        $doiManager = DoiManager::getInstance();
+
+        return $doiManager->getDuplicateDoiValues();
+    }
+}
diff --git a/library/Application/Document/DocumentDiff.php b/library/Application/Document/DocumentDiff.php
new file mode 100644
index 000000000..e017bc369
--- /dev/null
+++ b/library/Application/Document/DocumentDiff.php
@@ -0,0 +1,241 @@
+<?php
+
+/**
+ * This file is part of OPUS. The software OPUS has been originally developed
+ * at the University of Stuttgart with funding from the German Research Net,
+ * the Federal Department of Higher Education and Research and the Ministry
+ * of Science, Research and the Arts of the State of Baden-Wuerttemberg.
+ *
+ * OPUS 4 is a complete rewrite of the original OPUS software and was developed
+ * by the Stuttgart University Library, the Library Service Center
+ * Baden-Wuerttemberg, the Cooperative Library Network Berlin-Brandenburg,
+ * the Saarland University and State Library, the Saxon State Library -
+ * Dresden State and University Library, the Bielefeld University Library and
+ * the University Library of Hamburg University of Technology with funding from
+ * the German Research Foundation and the European Regional Development Fund.
+ *
+ * LICENCE
+ * OPUS is free software; you can redistribute it and/or modify it under the
+ * terms of the GNU General Public License as published by the Free Software
+ * Foundation; either version 2 of the Licence, or any later version.
+ * OPUS is distributed in the hope that it will be useful, but WITHOUT ANY
+ * WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
+ * FOR A PARTICULAR PURPOSE. See the GNU General Public License for more
+ * details. You should have received a copy of the GNU General Public License
+ * along with OPUS; if not, write to the Free Software Foundation, Inc., 51
+ * Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ *
+ * @copyright   Copyright (c) 2023, OPUS 4 development team
+ * @license     http://www.gnu.org/licenses/gpl.html General Public License
+ */
+
+use Opus\Common\Document;
+use Symfony\Component\Console\Output\ConsoleOutput;
+use Symfony\Component\Console\Output\OutputInterface;
+
+/**
+ * Shows differences between two or more documents.
+ *
+ * TODO more testing
+ * TODO return diff report that can be rendered in different ways (console, HTML)
+ * TODO add detailed diff for text values and multiline texts
+ * TODO add detailed comparison of persons (recognize same persons in different order, changes in person metadata)
+ * TODO ignore order of collections, licences (order shouldn't matter)
+ */
+class Application_Document_DocumentDiff
+{
+    /** @var OutputInterface */
+    private $output;
+
+    /**
+     * @param int[] $docIds
+     */
+    public function diff($docIds)
+    {
+        $metadata = [];
+        $keys     = [];
+
+        foreach ($docIds as $docId) {
+            $doc              = Document::get($docId);
+            $docData          = $doc->toArray();
+            $keys             = array_unique(array_merge($keys, array_keys($docData)));
+            $metadata[$docId] = $docData;
+        }
+
+        $maxDocId  = max(array_keys($metadata));
+        $maxDigits = strlen((string) $maxDocId);
+
+        sort($keys);
+
+        $differences = [];
+
+        foreach ($keys as $key) {
+            $values = [];
+
+            foreach ($metadata as $docId => $docData) {
+                if (isset($docData[$key])) {
+                    $value = $docData[$key];
+                } else {
+                    $value = null;
+                }
+
+                $values[$docId] = $value;
+            }
+
+            $previousValue = null;
+            $firstValue    = true;
+
+            foreach ($values as $value) {
+                if ($firstValue) {
+                    $previousValue = $value;
+                    $firstValue    = false;
+                } else {
+                    if ($previousValue !== $value) {
+                        $differences[$key] = $values;
+                    }
+                }
+            }
+        }
+
+        $diffKeys = array_keys($differences);
+
+        $output = $this->getOutput();
+
+        foreach ($diffKeys as $key) {
+            $output->writeln("Field: <fg=yellow;options=bold>$key</>");
+            $values          = $differences[$key];
+            $field           = $doc->getField($key);
+            $valueModelClass = $field->getValueModelClass();
+            $linkModelClass  = $field->getLinkModelClass();
+
+            foreach ($values as $docId => $value) {
+                $output->writeln('------------------------------------------------------------');
+                if (! is_array($value)) {
+                    $line = sprintf(" <fg=green>%{$maxDigits}d</>: %s", $docId, $value);
+                    $output->writeln($line);
+                } else {
+                    if (count($value) > 0) {
+                        if ($valueModelClass !== null) {
+                            if ($field->hasMultipleValues()) {
+                                foreach ($value as $subValue) {
+                                    $this->renderModel($key, $valueModelClass, $linkModelClass, $subValue, $docId, $maxDigits);
+                                }
+                            } else {
+                                $this->renderModel($key, $valueModelClass, $linkModelClass, $value, $docId, $maxDigits);
+                            }
+                        }
+                    } else {
+                        $line = sprintf(" <fg=green>%{$maxDigits}d</>: %s", $docId, '-');
+                        $output->writeln($line);
+                    }
+                }
+            }
+            $output->writeln('------------------------------------------------------------');
+            $output->writeln('');
+        }
+    }
+
+    /**
+     * @param string      $key
+     * @param string|null $valueModelClass
+     * @param string|null $linkModelClass
+     * @param mixed       $value
+     * @param int         $docId
+     * @param int         $maxDigits
+     */
+    protected function renderModel($key, $valueModelClass, $linkModelClass, $value, $docId, $maxDigits)
+    {
+        if (strpos($key, 'Person') === 0) {
+            unset($value['DateOfBirth']); // TODO date field causes problems with fromArray
+        }
+
+        if ($key === 'File') {
+            unset($value['ServerDateSubmitted']); // TODO date field causes problems with fromArray
+        }
+
+        if ($linkModelClass !== null) {
+            $model = $linkModelClass::fromArray($value);
+        } else {
+            $model = $valueModelClass::fromArray($value);
+        }
+
+        switch ($key) {
+            case 'Collection':
+                $displayName = $model->getDisplayName();
+                break;
+
+            case 'EmbargoDate':
+            case 'CompletedDate':
+            case 'PublishedDate':
+            case 'ServerDatePublished':
+            case 'ServerDateCreated':
+            case 'ServerDateModified':
+            case 'ThesisDateAccepted':
+                $displayName = (string) $model;
+                break;
+
+            case 'Enrichment':
+                $displayName = '<fg=magenta>' . $model->getKeyName() . '</>: ' . $model->getValue();
+                break;
+
+            case 'Identifier':
+                $displayName = '<fg=magenta>' . $model->getType() . '</>: ' . $model->getValue();
+                break;
+
+            case 'TitleMain':
+            case 'TitleAbstract':
+            case 'TitleAdditional':
+            case 'TitleParent':
+            case 'TitleSub':
+                $displayName = '(<fg=magenta>' . $model->getLanguage() . '</>) ' . $model->getValue();
+                break;
+
+            case 'Note':
+                $displayName = $model->getMessage();
+                break;
+
+            case 'Patent':
+                $displayName = $model->getNumber() . ' - ' . $model->getApplication();
+                break;
+
+            case 'Subject':
+                $displayName = $model->getValue() . ' (<fg=magenta>' . $model->getType() . '</>)';
+                break;
+
+            case 'Person':
+                $displayName = $model->getDisplayName() . ' (<fg=magenta>' . $model->getRole() . '</>)';
+                break;
+
+            case 'File':
+                $displayName = $model->getLabel() . ' (<fg=magenta>' . $model->getMimeType() . '</>)';
+                break;
+
+            default:
+                $displayName = $model->getDisplayName();
+        }
+
+        $line = sprintf(" <fg=green>%{$maxDigits}d</>: %s", $docId, $displayName);
+        $this->getOutput()->writeln($line);
+    }
+
+    /**
+     * @param OutputInterface $output
+     * @return $this
+     */
+    public function setOutput($output)
+    {
+        $this->output = $output;
+        return $this;
+    }
+
+    /**
+     * @return OutputInterface
+     */
+    public function getOutput()
+    {
+        if ($this->output === null) {
+            $this->output = new ConsoleOutput();
+        }
+        return $this->output;
+    }
+}
diff --git a/library/Application/Document/DuplicateFinder.php b/library/Application/Document/DuplicateFinder.php
new file mode 100644
index 000000000..fac639b60
--- /dev/null
+++ b/library/Application/Document/DuplicateFinder.php
@@ -0,0 +1,370 @@
+<?php
+
+/**
+ * This file is part of OPUS. The software OPUS has been originally developed
+ * at the University of Stuttgart with funding from the German Research Net,
+ * the Federal Department of Higher Education and Research and the Ministry
+ * of Science, Research and the Arts of the State of Baden-Wuerttemberg.
+ *
+ * OPUS 4 is a complete rewrite of the original OPUS software and was developed
+ * by the Stuttgart University Library, the Library Service Center
+ * Baden-Wuerttemberg, the Cooperative Library Network Berlin-Brandenburg,
+ * the Saarland University and State Library, the Saxon State Library -
+ * Dresden State and University Library, the Bielefeld University Library and
+ * the University Library of Hamburg University of Technology with funding from
+ * the German Research Foundation and the European Regional Development Fund.
+ *
+ * LICENCE
+ * OPUS is free software; you can redistribute it and/or modify it under the
+ * terms of the GNU General Public License as published by the Free Software
+ * Foundation; either version 2 of the Licence, or any later version.
+ * OPUS is distributed in the hope that it will be useful, but WITHOUT ANY
+ * WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
+ * FOR A PARTICULAR PURPOSE. See the GNU General Public License for more
+ * details. You should have received a copy of the GNU General Public License
+ * along with OPUS; if not, write to the Free Software Foundation, Inc., 51
+ * Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ *
+ * @copyright   Copyright (c) 2023, OPUS 4 development team
+ * @license     http://www.gnu.org/licenses/gpl.html General Public License
+ */
+
+use Opus\Common\Config;
+use Opus\Common\Document;
+use Opus\Common\DocumentInterface;
+use Opus\Common\Repository;
+use Symfony\Component\Console\Helper\ProgressBar;
+use Symfony\Component\Console\Output\ConsoleOutput;
+use Symfony\Component\Console\Output\OutputInterface;
+
+/**
+ * Removes duplicate documents identified by DOI.
+ *
+ * TODO rename function to findDuplicateDocuments
+ * TODO make "action" optional (and configurable in the long run)
+ * TODO logging
+ * TODO dry-run -> need to generate a report (CSV)
+ */
+class Application_Document_DuplicateFinder
+{
+    /** @var OutputInterface */
+    private $output;
+
+    /** @var ProgressBar */
+    private $progressBar;
+
+    /** @var resource */
+    private $csvFile;
+
+    /** @var bool */
+    private $dryRun;
+
+    /** @var string */
+    private $fromDate;
+
+    /** @var string */
+    private $untilDate;
+
+    /** @var bool */
+    private $remove;
+
+    /**
+     * @param string[] $listOfDoi
+     */
+    public function removeDuplicateDocuments($listOfDoi)
+    {
+        $progressBar = $this->getProgressBar();
+        foreach ($listOfDoi as $doi) {
+            $this->removeDuplicateDocument($doi);
+            if ($progressBar) {
+                $progressBar->advance();
+            }
+        }
+    }
+
+    /**
+     * @param string $doi
+     */
+    public function removeDuplicateDocument($doi)
+    {
+        $output = $this->getOutput();
+
+        $output->write("Checking $doi ... ", false, OutputInterface::VERBOSITY_VERBOSE);
+
+        $docIds = $this->findDocuments($doi);
+
+        $docCount = count($docIds);
+
+        $output->write("found {$docCount} document(s)", false, OutputInterface::VERBOSITY_VERBOSE);
+
+        if (count($docIds) > 1) {
+            if ($output->isVerbose()) {
+                $output->write(' - ' . implode(', ', $docIds) . ' ');
+            }
+
+            foreach ($docIds as $docId) {
+                $doc = Document::get($docId);
+                $this->writeCsv($doi, $doc);
+            }
+
+            // TODO log if more than 2 documents were found
+            $doc         = $this->getNewestDocument($docIds);
+            $docId       = $doc->getId();
+            $serverState = $doc->getServerState();
+
+            if ($doc->getServerState() === Document::STATE_UNPUBLISHED) {
+                if ($output->isVerbose()) {
+                    $output->write("- REMOVE document <fg=yellow>{$docId}</>");
+                }
+                $this->performAction($doc);
+            } else {
+                $output->write(
+                    "- KEEP document <fg=yellow>{$docId}</> in state '{$serverState}'",
+                    false,
+                    OutputInterface::VERBOSITY_VERBOSE
+                );
+            }
+        } else {
+            $this->writeCsv($doi);
+        }
+
+        $output->writeln('', OutputInterface::VERBOSITY_VERBOSE);
+    }
+
+    /**
+     * @param string $doi
+     * @return int[]
+     */
+    public function findDocuments($doi)
+    {
+        $finder = Repository::getInstance()->getDocumentFinder();
+
+        $finder->setIdentifierValue('doi', $doi);
+
+        return $finder->getIds();
+    }
+
+    /**
+     * @param int[] $docIds
+     * @return DocumentInterface
+     */
+    public function getNewestDocument($docIds)
+    {
+        $doc = null;
+
+        foreach ($docIds as $docId) {
+            $nextDoc = Document::get($docId);
+            if ($doc === null) {
+                $doc = $nextDoc;
+            } else {
+                switch ($doc->getServerDateCreated()->compare($nextDoc->getServerDateCreated())) {
+                    case -1:
+                        $doc = $nextDoc;
+                        break;
+
+                    case 0:
+                        // if ServerDateCreated is the same keep the document with the higher database ID
+                        if ($doc->getId() < $nextDoc->getId()) {
+                            $doc = $nextDoc;
+                        }
+                        break;
+
+                    default:
+                        break;
+                }
+            }
+        }
+
+        return $doc;
+    }
+
+    /**
+     * @param string                 $doi
+     * @param DocumentInterface|null $doc
+     */
+    protected function writeCsv($doi, $doc = null)
+    {
+        $csvFile = $this->getCsvFile();
+
+        if ($csvFile !== null) {
+            if ($doc !== null) {
+                $baseLink    = $this->getBaseLink();
+                $docId       = $doc->getId();
+                $dateCreated = $doc->getServerDateCreated()->getDateTime();
+
+                $data = [
+                    $doi,
+                    $docId,
+                    "<$baseLink/$docId>",
+                    $dateCreated->format('Y-m-d'),
+                    $doc->getServerState(),
+                ];
+            } else {
+                $data = [
+                    $doi,
+                    'NOT_FOUND',
+                    '',
+                    '',
+                    '',
+                ];
+            }
+
+            fputcsv($csvFile, $data);
+        }
+    }
+
+    /**
+     * @return string|null
+     */
+    protected function getBaseLink()
+    {
+        $config = Config::get();
+
+        if (isset($config->url)) {
+            $url = $config->url ?? '';
+            return rtrim($url, "/ \n\r\t\v\x00");
+        }
+
+        return null;
+    }
+
+    /**
+     * @param DocumentInterface $doc
+     *
+     * TODO move action into separate classes for different actions (report, mark, delete, ...)
+     */
+    protected function performAction($doc)
+    {
+        if (! $this->isDryRunEnabled() && $this->isRemoveEnabled()) {
+            $doc->delete();
+        }
+    }
+
+    /**
+     * @param bool $enabled
+     */
+    public function setDryRunEnabled($enabled)
+    {
+        $this->dryRun = $enabled;
+    }
+
+    /**
+     * @return bool
+     */
+    public function isDryRunEnabled()
+    {
+        return $this->dryRun;
+    }
+
+    /**
+     * @param OutputInterface $output
+     */
+    public function setOutput($output)
+    {
+        $this->output = $output;
+    }
+
+    /**
+     * @return OutputInterface
+     */
+    public function getOutput()
+    {
+        if ($this->output === null) {
+            $this->output = new ConsoleOutput();
+        }
+        return $this->output;
+    }
+
+    /**
+     * @param string $fromDate
+     * @return $this
+     */
+    public function setFromDate($fromDate)
+    {
+        $this->fromDate = $fromDate;
+
+        return $this;
+    }
+
+    /**
+     * @return string
+     */
+    public function getFromDate()
+    {
+        return $this->fromDate;
+    }
+
+    /**
+     * @param string $untilDate
+     * @return $this
+     */
+    public function setUntilDate($untilDate)
+    {
+        $this->untilDate = $untilDate;
+
+        return $this;
+    }
+
+    /**
+     * @return string
+     */
+    public function getUntilDate()
+    {
+        return $this->untilDate;
+    }
+
+    /**
+     * @param bool $enabled
+     * @return $this
+     */
+    public function setRemoveEnabled($enabled)
+    {
+        $this->remove = $enabled;
+
+        return $this;
+    }
+
+    /**
+     * @return bool
+     */
+    public function isRemoveEnabled()
+    {
+        return $this->remove;
+    }
+
+    /**
+     * @param resource $csvFile
+     * @return $this
+     */
+    public function setCsvFile($csvFile)
+    {
+        $this->csvFile = $csvFile;
+        return $this;
+    }
+
+    /**
+     * @return resource|null
+     */
+    public function getCsvFile()
+    {
+        return $this->csvFile;
+    }
+
+    /**
+     * @param ProgressBar|null $progressBar
+     * @return $this
+     */
+    public function setProgressBar($progressBar)
+    {
+        $this->progressBar = $progressBar;
+        return $this;
+    }
+
+    /**
+     * @return ProgressBar|null
+     */
+    public function getProgressBar()
+    {
+        return $this->progressBar;
+    }
+}
diff --git a/tests/library/Application/Console/Document/DiffCommandTest.php b/tests/library/Application/Console/Document/DiffCommandTest.php
new file mode 100644
index 000000000..dc4d0ceb5
--- /dev/null
+++ b/tests/library/Application/Console/Document/DiffCommandTest.php
@@ -0,0 +1,41 @@
+<?php
+
+/**
+ * This file is part of OPUS. The software OPUS has been originally developed
+ * at the University of Stuttgart with funding from the German Research Net,
+ * the Federal Department of Higher Education and Research and the Ministry
+ * of Science, Research and the Arts of the State of Baden-Wuerttemberg.
+ *
+ * OPUS 4 is a complete rewrite of the original OPUS software and was developed
+ * by the Stuttgart University Library, the Library Service Center
+ * Baden-Wuerttemberg, the Cooperative Library Network Berlin-Brandenburg,
+ * the Saarland University and State Library, the Saxon State Library -
+ * Dresden State and University Library, the Bielefeld University Library and
+ * the University Library of Hamburg University of Technology with funding from
+ * the German Research Foundation and the European Regional Development Fund.
+ *
+ * LICENCE
+ * OPUS is free software; you can redistribute it and/or modify it under the
+ * terms of the GNU General Public License as published by the Free Software
+ * Foundation; either version 2 of the Licence, or any later version.
+ * OPUS is distributed in the hope that it will be useful, but WITHOUT ANY
+ * WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
+ * FOR A PARTICULAR PURPOSE. See the GNU General Public License for more
+ * details. You should have received a copy of the GNU General Public License
+ * along with OPUS; if not, write to the Free Software Foundation, Inc., 51
+ * Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ *
+ * @copyright   Copyright (c) 2023, OPUS 4 development team
+ * @license     http://www.gnu.org/licenses/gpl.html General Public License
+ */
+
+class Application_Console_Document_DiffCommandTest extends ControllerTestCase
+{
+    /** @var string[] */
+    protected $additionalResources = ['database'];
+
+    public function testCompareDoiDocuments()
+    {
+        $this->markTestIncomplete('TODO testing');
+    }
+}
diff --git a/tests/library/Application/Console/Document/DuplicatesCommandTest.php b/tests/library/Application/Console/Document/DuplicatesCommandTest.php
new file mode 100644
index 000000000..c795505dd
--- /dev/null
+++ b/tests/library/Application/Console/Document/DuplicatesCommandTest.php
@@ -0,0 +1,41 @@
+<?php
+
+/**
+ * This file is part of OPUS. The software OPUS has been originally developed
+ * at the University of Stuttgart with funding from the German Research Net,
+ * the Federal Department of Higher Education and Research and the Ministry
+ * of Science, Research and the Arts of the State of Baden-Wuerttemberg.
+ *
+ * OPUS 4 is a complete rewrite of the original OPUS software and was developed
+ * by the Stuttgart University Library, the Library Service Center
+ * Baden-Wuerttemberg, the Cooperative Library Network Berlin-Brandenburg,
+ * the Saarland University and State Library, the Saxon State Library -
+ * Dresden State and University Library, the Bielefeld University Library and
+ * the University Library of Hamburg University of Technology with funding from
+ * the German Research Foundation and the European Regional Development Fund.
+ *
+ * LICENCE
+ * OPUS is free software; you can redistribute it and/or modify it under the
+ * terms of the GNU General Public License as published by the Free Software
+ * Foundation; either version 2 of the Licence, or any later version.
+ * OPUS is distributed in the hope that it will be useful, but WITHOUT ANY
+ * WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
+ * FOR A PARTICULAR PURPOSE. See the GNU General Public License for more
+ * details. You should have received a copy of the GNU General Public License
+ * along with OPUS; if not, write to the Free Software Foundation, Inc., 51
+ * Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ *
+ * @copyright   Copyright (c) 2023, OPUS 4 development team
+ * @license     http://www.gnu.org/licenses/gpl.html General Public License
+ */
+
+class Application_Console_Document_DuplicatesCommandTest extends ControllerTestCase
+{
+    /** @var string[] */
+    protected $additionalResources = ['database'];
+
+    public function testFindDuplicates()
+    {
+        $this->markTestIncomplete('TODO testing');
+    }
+}
diff --git a/tests/library/Application/Document/DocumentDiffTest.php b/tests/library/Application/Document/DocumentDiffTest.php
new file mode 100644
index 000000000..85fe204fd
--- /dev/null
+++ b/tests/library/Application/Document/DocumentDiffTest.php
@@ -0,0 +1,38 @@
+<?php
+
+/**
+ * This file is part of OPUS. The software OPUS has been originally developed
+ * at the University of Stuttgart with funding from the German Research Net,
+ * the Federal Department of Higher Education and Research and the Ministry
+ * of Science, Research and the Arts of the State of Baden-Wuerttemberg.
+ *
+ * OPUS 4 is a complete rewrite of the original OPUS software and was developed
+ * by the Stuttgart University Library, the Library Service Center
+ * Baden-Wuerttemberg, the Cooperative Library Network Berlin-Brandenburg,
+ * the Saarland University and State Library, the Saxon State Library -
+ * Dresden State and University Library, the Bielefeld University Library and
+ * the University Library of Hamburg University of Technology with funding from
+ * the German Research Foundation and the European Regional Development Fund.
+ *
+ * LICENCE
+ * OPUS is free software; you can redistribute it and/or modify it under the
+ * terms of the GNU General Public License as published by the Free Software
+ * Foundation; either version 2 of the Licence, or any later version.
+ * OPUS is distributed in the hope that it will be useful, but WITHOUT ANY
+ * WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
+ * FOR A PARTICULAR PURPOSE. See the GNU General Public License for more
+ * details. You should have received a copy of the GNU General Public License
+ * along with OPUS; if not, write to the Free Software Foundation, Inc., 51
+ * Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ *
+ * @copyright   Copyright (c) 2023, OPUS 4 development team
+ * @license     http://www.gnu.org/licenses/gpl.html General Public License
+ */
+
+class Application_Document_DocumentDiffTest extends ControllerTestCase
+{
+    public function testRenderModelLicence()
+    {
+        $this->markTestIncomplete('TODO testing');
+    }
+}
diff --git a/tests/library/Application/Document/DuplicateFinderTest.php b/tests/library/Application/Document/DuplicateFinderTest.php
new file mode 100644
index 000000000..606cca9c4
--- /dev/null
+++ b/tests/library/Application/Document/DuplicateFinderTest.php
@@ -0,0 +1,162 @@
+<?php
+
+/**
+ * This file is part of OPUS. The software OPUS has been originally developed
+ * at the University of Stuttgart with funding from the German Research Net,
+ * the Federal Department of Higher Education and Research and the Ministry
+ * of Science, Research and the Arts of the State of Baden-Wuerttemberg.
+ *
+ * OPUS 4 is a complete rewrite of the original OPUS software and was developed
+ * by the Stuttgart University Library, the Library Service Center
+ * Baden-Wuerttemberg, the Cooperative Library Network Berlin-Brandenburg,
+ * the Saarland University and State Library, the Saxon State Library -
+ * Dresden State and University Library, the Bielefeld University Library and
+ * the University Library of Hamburg University of Technology with funding from
+ * the German Research Foundation and the European Regional Development Fund.
+ *
+ * LICENCE
+ * OPUS is free software; you can redistribute it and/or modify it under the
+ * terms of the GNU General Public License as published by the Free Software
+ * Foundation; either version 2 of the Licence, or any later version.
+ * OPUS is distributed in the hope that it will be useful, but WITHOUT ANY
+ * WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
+ * FOR A PARTICULAR PURPOSE. See the GNU General Public License for more
+ * details. You should have received a copy of the GNU General Public License
+ * along with OPUS; if not, write to the Free Software Foundation, Inc., 51
+ * Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ *
+ * @copyright   Copyright (c) 2023, OPUS 4 development team
+ * @license     http://www.gnu.org/licenses/gpl.html General Public License
+ */
+
+use Opus\Common\Document;
+use Opus\Common\Model\NotFoundException;
+use Symfony\Component\Console\Output\NullOutput;
+
+/**
+ * TODO create fixture helper class to create N test documents and get their IDs
+ *      additionally an array with data could be provided to be used for creating
+ *      the test documents
+ */
+class Application_Document_DuplicateFinderTest extends ControllerTestCase
+{
+    /** @var int */
+    private $docId1;
+
+    /** @var int */
+    private $docId2;
+
+    /** @var int */
+    private $docId3;
+
+    /** @var Application_Document_DuplicateFinder */
+    private $helper;
+
+    /** @var string[] */
+    protected $additionalResources = ['database'];
+
+    public function setUp(): void
+    {
+        parent::setUp();
+
+        $this->helper = new Application_Document_DuplicateFinder();
+        $this->helper->setOutput(new NullOutput());
+        $this->helper->setRemoveEnabled(true);
+
+        $this->setupTestDocuments();
+    }
+
+    public function testFindDocuments()
+    {
+        $helper = $this->helper;
+
+        $doi = '10.1000/182';
+
+        $docIds = $helper->findDocuments($doi);
+
+        $this->assertCount(2, $docIds);
+    }
+
+    public function testFindDocumentsUnknownDoi()
+    {
+        $helper = $this->helper;
+
+        $doi = '10.1000/282';
+
+        $docIds = $helper->findDocuments($doi);
+
+        $this->assertCount(0, $docIds);
+    }
+
+    public function testRemoveNewerDuplicateDocument()
+    {
+        $this->helper->removeDuplicateDocument('10.1000/182');
+
+        Document::get($this->docId3);
+
+        $this->expectException(NotFoundException::class);
+
+        Document::get($this->docId2);
+    }
+
+    public function testDoNotRemoveNewerDuplicateDocumentIfDryRunEnabled()
+    {
+        $helper = $this->helper;
+
+        $helper->setDryRunEnabled(true);
+        $helper->removeDuplicateDocument('10.1000/182');
+
+        Document::get($this->docId2);
+        Document::get($this->docId1);
+    }
+
+    public function testRemoveOnlyUnpublishedDocuments()
+    {
+        $doc = Document::get($this->docId2);
+        $doc->setServerState(Document::STATE_INPROGRESS);
+        $doc->store();
+
+        $this->helper->removeDuplicateDocument('10.1000/182');
+
+        Document::get($this->docId2);
+        Document::get($this->docId1);
+        Document::get($this->docId3);
+    }
+
+    public function testGetNewestDocument()
+    {
+        $docIds = [$this->docId1, $this->docId2];
+
+        $doc = $this->helper->getNewestDocument($docIds);
+
+        $this->assertNotNull($doc);
+        $this->assertEquals($this->docId2, $doc->getId());
+
+        $docIds = [$this->docId2, $this->docId1];
+
+        $doc = $this->helper->getNewestDocument($docIds);
+
+        $this->assertNotNull($doc);
+        $this->assertEquals($this->docId2, $doc->getId());
+    }
+
+    protected function setupTestDocuments()
+    {
+        $doc = $this->createTestDocument();
+        $doi = $doc->addIdentifierDoi();
+        $doi->setValue('10.1000/182');
+        $this->docId1 = $doc->store();
+
+        sleep(1);
+
+        $doc = $this->createTestDocument();
+        $doi = $doc->addIdentifierDoi();
+        $doi->setValue('10.1000/182');
+        $this->docId2 = $doc->store();
+
+        $doc = $this->createTestDocument();
+        $doi = $doc->addIdentifierDoi();
+        $doi->setValue('10.1000/183');
+        $this->docId3 = $doc->store();
+    }
+}