diff --git a/UPGRADE.md b/UPGRADE.md
index 9de8af04d38..45c222f3caf 100644
--- a/UPGRADE.md
+++ b/UPGRADE.md
@@ -1,3 +1,8 @@
+# Upgrade to 2.21
+
+Using the same class several times in a discriminator map is deprecated.
+In 4.0, this will be an error.
+
 # Upgrade to 2.20
 
 ## Add `Doctrine\ORM\Query\OutputWalker` interface, deprecate `Doctrine\ORM\Query\SqlWalker::getExecutor()`
diff --git a/src/Mapping/ClassMetadataInfo.php b/src/Mapping/ClassMetadataInfo.php
index cb17b8983d4..14a7d665349 100644
--- a/src/Mapping/ClassMetadataInfo.php
+++ b/src/Mapping/ClassMetadataInfo.php
@@ -23,7 +23,9 @@
 use ReflectionProperty;
 use RuntimeException;
 
+use function array_count_values;
 use function array_diff;
+use function array_filter;
 use function array_flip;
 use function array_intersect;
 use function array_keys;
@@ -37,6 +39,7 @@
 use function enum_exists;
 use function explode;
 use function gettype;
+use function implode;
 use function in_array;
 use function interface_exists;
 use function is_array;
@@ -3207,6 +3210,22 @@ final public function getDiscriminatorColumn(): array
      */
     public function setDiscriminatorMap(array $map)
     {
+        if (count(array_flip($map)) !== count($map)) {
+            Deprecation::trigger(
+                'doctrine/orm',
+                'https://github.com/doctrine/orm/issues/3519',
+                <<<'DEPRECATION'
+                Mapping a class to multiple discriminator values is deprecated,
+                and the discriminator mapping of %s contains duplicate values
+                for the following discriminator values: %s.
+                DEPRECATION,
+                $this->name,
+                implode(', ', array_keys(array_filter(array_count_values($map), static function (int $value): bool {
+                    return $value > 1;
+                })))
+            );
+        }
+
         foreach ($map as $value => $className) {
             $this->addDiscriminatorMapClass($value, $className);
         }
diff --git a/tests/Tests/ORM/Mapping/ClassMetadataTest.php b/tests/Tests/ORM/Mapping/ClassMetadataTest.php
index fcd4cffc35f..4cd3abd06d8 100644
--- a/tests/Tests/ORM/Mapping/ClassMetadataTest.php
+++ b/tests/Tests/ORM/Mapping/ClassMetadataTest.php
@@ -1408,6 +1408,14 @@ public function testClassNameMappingDiscriminatorValue(): void
             $xmlElement->children()->{'discriminator-map'}->{'discriminator-mapping'}[0]->attributes()['value']
         );
     }
+
+    public function testDiscriminatorMapWithSameClassMultipleTimesDeprecated(): void
+    {
+        $this->expectDeprecationWithIdentifier('https://github.com/doctrine/orm/issues/3519');
+
+        $cm = new ClassMetadata(CMS\CmsUser::class);
+        $cm->setDiscriminatorMap(['foo' => CMS\CmsUser::class, 'bar' => CMS\CmsUser::class]);
+    }
 }
 
 /** @MappedSuperclass */