[GH-3519] Deprecate passing the same class with different discriminator values.

UPGRADE.md

@@ -1,3 +1,8 @@
+# Upgrade to 3.4
+
+Using the same class several times in a discriminator map is deprecated.
+In 4.0, this will be an error.
+
 # Upgrade to 3.3
 
 ## Deprecate `DatabaseDriver`
@@ -737,7 +742,7 @@ Use `toIterable()` instead.
 
 Output walkers should implement the new `\Doctrine\ORM\Query\OutputWalker` interface and create
 `Doctrine\ORM\Query\Exec\SqlFinalizer` instances instead of `Doctrine\ORM\Query\Exec\AbstractSqlExecutor`s.
-The output walker must not base its workings on the query `firstResult`/`maxResult` values, so that the 
+The output walker must not base its workings on the query `firstResult`/`maxResult` values, so that the
 `SqlFinalizer` can be kept in the query cache and used regardless of the actual `firstResult`/`maxResult` values.
 Any operation dependent on `firstResult`/`maxResult` should take place within the `SqlFinalizer::createExecutor()`
 method. Details can be found at https://github.com/doctrine/orm/pull/11188.
@@ -750,7 +755,7 @@ change in behavior.
 
 Progress on this is tracked at https://github.com/doctrine/orm/issues/11624 .
 
-## PARTIAL DQL syntax is undeprecated 
+## PARTIAL DQL syntax is undeprecated
 
 Use of the PARTIAL keyword is not deprecated anymore in DQL, because we will be
 able to support PARTIAL objects with PHP 8.4 Lazy Objects and

src/Mapping/ClassMetadata.php

@@ -25,7 +25,10 @@
 use Stringable;
 
 use function array_column;
+use function array_count_values;
 use function array_diff;
+use function array_filter;
+use function array_flip;
 use function array_intersect;
 use function array_key_exists;
 use function array_keys;
@@ -39,6 +42,7 @@
 use function defined;
 use function enum_exists;
 use function explode;
+use function implode;
 use function in_array;
 use function interface_exists;
 use function is_string;
@@ -2178,6 +2182,22 @@ final public function getDiscriminatorColumn(): DiscriminatorColumnMapping
      */
     public function setDiscriminatorMap(array $map): void
     {
+        if (count(array_flip($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);
         }

tests/Tests/ORM/Mapping/ClassMetadataTest.php

@@ -1101,6 +1101,14 @@ 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]