diff --git a/src/Bundle/ChillThirdPartyBundle/Service/ThirdpartyMergeService.php b/src/Bundle/ChillThirdPartyBundle/Service/ThirdpartyMergeService.php
index daea6627f..f50306a5b 100644
--- a/src/Bundle/ChillThirdPartyBundle/Service/ThirdpartyMergeService.php
+++ b/src/Bundle/ChillThirdPartyBundle/Service/ThirdpartyMergeService.php
@@ -101,8 +101,11 @@ readonly class ThirdpartyMergeService
                 $prefix = null !== ($assoc['joinTable']['schema'] ?? null) ? $assoc['joinTable']['schema'].'.' : '';
                 $joinColumn = $assoc['joinTable']['joinColumns'][0]['name']; // Note: joinColumns, not inverseJoinColumns
 
+                // Get the other column name to build proper duplicate check
+                $otherColumn = $assoc['joinTable']['inverseJoinColumns'][0]['name'];
+
                 $queries[] = [
-                    'sql' => "UPDATE {$prefix}{$joinTable} SET {$joinColumn} = :toKeep WHERE {$joinColumn} = :toDelete",
+                    'sql' => "UPDATE {$prefix}{$joinTable} SET {$joinColumn} = :toKeep WHERE {$joinColumn} = :toDelete AND NOT EXISTS (SELECT 1 FROM {$prefix}{$joinTable} AS t2 WHERE t2.{$joinColumn} = :toKeep AND t2.{$otherColumn} = {$prefix}{$joinTable}.{$otherColumn})",
                     'params' => ['toKeep' => $toKeep->getId(), 'toDelete' => $toDelete->getId()],
                 ];
 
diff --git a/src/Bundle/ChillThirdPartyBundle/Tests/Service/ThirdpartyMergeServiceTest.php b/src/Bundle/ChillThirdPartyBundle/Tests/Service/ThirdpartyMergeServiceTest.php
index 4b2819751..4d8afbb7d 100644
--- a/src/Bundle/ChillThirdPartyBundle/Tests/Service/ThirdpartyMergeServiceTest.php
+++ b/src/Bundle/ChillThirdPartyBundle/Tests/Service/ThirdpartyMergeServiceTest.php
@@ -12,6 +12,7 @@ declare(strict_types=1);
 namespace Chill\ThirdPartyBundle\Tests\Service;
 
 use Chill\ActivityBundle\Entity\Activity;
+use Chill\MainBundle\Entity\Center;
 use Chill\ThirdPartyBundle\Entity\ThirdParty;
 use Chill\ThirdPartyBundle\Entity\ThirdPartyCategory;
 use Chill\ThirdPartyBundle\Service\ThirdpartyMergeService;
@@ -47,19 +48,20 @@ class ThirdpartyMergeServiceTest extends KernelTestCase
         $toDelete->setName('Thirdparty to delete');
         $this->em->persist($toDelete);
 
-        // Create a related entity with TO_ONE relation (thirdparty parent)
+        // Create a related entity with TO_ONE relation (thirdparty parent) - tests schema handling for TO_ONE
         $relatedToOneEntity = new ThirdParty();
         $relatedToOneEntity->setName('RelatedToOne thirdparty');
         $relatedToOneEntity->setParent($toDelete);
         $this->em->persist($relatedToOneEntity);
 
-        // Create a related entity with TO_MANY relation (thirdparty category)
+        // Create a related entity with MANY_TO_MANY relation (thirdparty category) - tests schema handling for MANY_TO_MANY where ThirdParty is target
         $thirdpartyCategory = new ThirdPartyCategory();
         $thirdpartyCategory->setName(['fr' => 'Thirdparty category']);
         $this->em->persist($thirdpartyCategory);
         $toDelete->addCategory($thirdpartyCategory);
         $this->em->persist($toDelete);
 
+        // Test MANY_TO_MANY relation from another bundle (Activity) - tests cross-bundle schema handling
         $activity = new Activity();
         $activity->setDate(new \DateTime());
         $activity->addThirdParty($toDelete);
@@ -73,14 +75,55 @@ class ThirdpartyMergeServiceTest extends KernelTestCase
         $this->em->refresh($relatedToOneEntity);
 
         // Check that references were updated
-        $this->assertEquals($toKeep->getId(), $relatedToOneEntity->getParent()->getId(), 'The parent thirdparty was succesfully merged');
 
+        // Test TO_ONE relation in chill_3party schema was properly handled
+        $this->assertEquals($toKeep->getId(), $relatedToOneEntity->getParent()->getId(), 'The parent thirdparty in chill_3party schema was successfully merged');
+
+        // Test MANY_TO_MANY relation in chill_3party schema was properly handled
         $updatedRelatedManyEntity = $this->em->find(ThirdPartyCategory::class, $thirdpartyCategory->getId());
-        $this->assertContains($updatedRelatedManyEntity, $toKeep->getCategories(), 'The thirdparty category was found in the toKeep entity');
+        $this->assertContains($updatedRelatedManyEntity, $toKeep->getCategories(), 'The thirdparty category in chill_3party schema was found in the toKeep entity');
+
+        // Test MANY_TO_MANY relation from different schema (Activity bundle) was properly handled
+        $this->em->refresh($activity);
+        $this->assertContains($toKeep, $activity->getThirdParties(), 'The activity relation from different schema was successfully merged');
+        $this->assertNotContains($toDelete, $activity->getThirdParties(), 'The toDelete thirdparty was removed from activity relation');
 
         // Check that toDelete was removed
         $this->em->clear();
         $deletedThirdParty = $this->em->find(ThirdParty::class, $toDelete->getId());
-        $this->assertNull($deletedThirdParty);
+        $this->assertNull($deletedThirdParty, 'The toDelete thirdparty was successfully removed');
+    }
+
+    public function testMergeWithSharedCenterDoesNotCauseUniqueConstraintViolation(): void
+    {
+        // Create a center that will be shared by both thirdparties
+        $sharedCenter = new Center();
+        $sharedCenter->setName('Shared Center');
+        $this->em->persist($sharedCenter);
+
+        // Create ThirdParty entities
+        $toKeep = new ThirdParty();
+        $toKeep->setName('Thirdparty to keep');
+        $toKeep->addCenter($sharedCenter); // Both thirdparties linked to same center
+        $this->em->persist($toKeep);
+
+        $toDelete = new ThirdParty();
+        $toDelete->setName('Thirdparty to delete');
+        $toDelete->addCenter($sharedCenter); // Both thirdparties linked to same center
+        $this->em->persist($toDelete);
+
+        $this->em->flush();
+
+        // This should not throw a unique constraint violation
+        $this->service->merge($toKeep, $toDelete);
+
+        // Verify that toKeep still has the shared center
+        $this->em->refresh($toKeep);
+        $this->assertContains($sharedCenter, $toKeep->getCenters(), 'The shared center is still linked to the kept thirdparty');
+
+        // Verify that toDelete was removed
+        $this->em->clear();
+        $deletedThirdParty = $this->em->find(ThirdParty::class, $toDelete->getId());
+        $this->assertNull($deletedThirdParty, 'The toDelete thirdparty was successfully removed');
     }
 }