diff --git a/src/Bundle/ChillThirdPartyBundle/Controller/ThirdpartyDuplicateController.php b/src/Bundle/ChillThirdPartyBundle/Controller/ThirdpartyDuplicateController.php
index a7fb87458..0c0671aaf 100644
--- a/src/Bundle/ChillThirdPartyBundle/Controller/ThirdpartyDuplicateController.php
+++ b/src/Bundle/ChillThirdPartyBundle/Controller/ThirdpartyDuplicateController.php
@@ -21,7 +21,7 @@ use Symfony\Component\Routing\Annotation\Route;
 
 class ThirdpartyDuplicateController extends AbstractController
 {
-    public function __construct(private ThirdPartyRepository $thirdPartyRepository, private ThirdpartyMergeService $thirdPartyMergeService) {}
+    public function __construct(private readonly ThirdPartyRepository $thirdPartyRepository, private readonly ThirdpartyMergeService $thirdPartyMergeService) {}
 
     #[Route(path: '/{_locale}/3party/{thirdparty_id}/find-manually', name: 'chill_thirdparty_find_duplicate')]
     public function findManuallyDuplicateAction(mixed $thirdparty_id, Request $request)
@@ -41,10 +41,6 @@ class ThirdpartyDuplicateController extends AbstractController
         if ($form->isSubmitted() && $form->isValid()) {
             $thirdparty2 = $form->get('thirdparty')->getData();
 
-            if (null === $thirdparty2) {
-                throw $this->createNotFoundException("Thirdparty with id {$thirdparty2->getId}() not".' found on this server');
-            }
-
             $direction = $form->get('direction')->getData();
 
             if ('starting' === $direction) {
diff --git a/src/Bundle/ChillThirdPartyBundle/Form/ThirdpartyFindDuplicateType.php b/src/Bundle/ChillThirdPartyBundle/Form/ThirdpartyFindDuplicateType.php
index 8b2534bca..e16cddf54 100644
--- a/src/Bundle/ChillThirdPartyBundle/Form/ThirdpartyFindDuplicateType.php
+++ b/src/Bundle/ChillThirdPartyBundle/Form/ThirdpartyFindDuplicateType.php
@@ -1,5 +1,14 @@
 <?php
 
+declare(strict_types=1);
+
+/*
+ * Chill is a software for social workers
+ *
+ * For the full copyright and license information, please view
+ * the LICENSE file that was distributed with this source code.
+ */
+
 namespace Chill\ThirdPartyBundle\Form;
 
 use Chill\ThirdPartyBundle\Form\Type\PickThirdpartyDynamicType;
@@ -28,5 +37,4 @@ class ThirdpartyFindDuplicateType extends AbstractType
     {
         return 'chill_thirdpartybundle_thirdparty_find_manually_duplicate';
     }
-
 }
diff --git a/src/Bundle/ChillThirdPartyBundle/Service/ThirdpartyMergeService.php b/src/Bundle/ChillThirdPartyBundle/Service/ThirdpartyMergeService.php
index dee1f987c..250144efd 100644
--- a/src/Bundle/ChillThirdPartyBundle/Service/ThirdpartyMergeService.php
+++ b/src/Bundle/ChillThirdPartyBundle/Service/ThirdpartyMergeService.php
@@ -63,7 +63,7 @@ class ThirdpartyMergeService
                     UPDATE chill_3party.third_party
                     SET {$column} = COALESCE((SELECT {$column} FROM chill_3party.third_party WHERE id = :toDelete), {$column})
                     WHERE id = :toKeep AND {$condition}",
-                'params' => ['toDelete' => $toDelete->getId(), 'toKeep' => $toKeep->getId()]
+                'params' => ['toDelete' => $toDelete->getId(), 'toKeep' => $toKeep->getId()],
             ];
         }
 
@@ -86,24 +86,40 @@ class ThirdpartyMergeService
                     continue;
                 }
 
-                if ($assoc['type'] & ClassMetadata::TO_ONE) {
+                if (ClassMetadata::TO_ONE === $assoc['type']) {
                     $joinColumn = $meta->getSingleAssociationJoinColumnName($assoc['fieldName']);
-                    $sql = "UPDATE {$tableName} SET {$joinColumn} = :toKeep WHERE {$joinColumn} = :toDelete";
 
-                    $queries[] = ['sql' => $sql, 'params' => ['toKeep' => $toKeep->getId(), 'toDelete' => $toDelete->getId()]];
-                }
+                    if (ThirdParty::class === $assoc['sourceEntity'] && 'parent_id' !== $joinColumn) {
+                        // TODO what with 'address_id' and 'civility_id'? This condition also contains columns like updatedBy_id which we want to ignore...
+                        continue;
+                    }
+
+                    $schemaPrefix = (ThirdParty::class === $assoc['sourceEntity'] && 'parent_id' === $joinColumn)
+                        ? 'chill_3party.'
+                        : '';
 
-                if ($assoc['type'] & ClassMetadata::TO_MANY && isset($assoc['joinTable'])) {
-                    $joinTable = $assoc['joinTable']['name'];
-                    $joinColumn = ThirdParty::class === $assoc['targetEntity']
-                        ? $assoc['joinTable']['inverseJoinColumns'][0]['name']
-                        : $assoc['joinTable']['joinColumns'][0]['name'];
-//TODO problem remaining for ManyToMany relations when UPDATE triggers a UNIQUE CONSTRAINT...in the case of Thirdparty -> thirdparty_category and thirdparty_center
                     $queries[] = [
-                        'sql' => "UPDATE {$joinTable} SET {$joinColumn} = :toKeep WHERE {$joinColumn} = :toDelete",
+                        'sql' => "UPDATE {$schemaPrefix}{$tableName} SET {$joinColumn} = :toKeep WHERE {$joinColumn} = :toDelete",
                         'params' => ['toKeep' => $toKeep->getId(), 'toDelete' => $toDelete->getId()],
                     ];
                 }
+
+                if (ClassMetadata::TO_MANY === $assoc['type'] && isset($assoc['joinTable'])) {
+
+                    $joinTable = $assoc['joinTable']['name'];
+                    $joinColumn = $assoc['joinTable']['joinColumns'][0]['name'];
+                    if ('thirdparty_id' === $joinColumn) {
+                        $queries[] = [
+                            'sql' => "DELETE FROM {$joinTable} WHERE {$joinColumn} = :toDelete",
+                            'params' => ['toDelete' => $toDelete->getId()],
+                        ];
+                    } else {
+                        $queries[] = [
+                            'sql' => "UPDATE {$joinTable} SET {$joinColumn} = :toKeep WHERE {$joinColumn} = :toDelete",
+                            '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 de11cea7a..42e7ec0d1 100644
--- a/src/Bundle/ChillThirdPartyBundle/Tests/Service/ThirdpartyMergeServiceTest.php
+++ b/src/Bundle/ChillThirdPartyBundle/Tests/Service/ThirdpartyMergeServiceTest.php
@@ -1,5 +1,14 @@
 <?php
 
+declare(strict_types=1);
+
+/*
+ * Chill is a software for social workers
+ *
+ * For the full copyright and license information, please view
+ * the LICENSE file that was distributed with this source code.
+ */
+
 namespace Chill\ThirdPartyBundle\Tests\Service;
 
 use Chill\ActivityBundle\Entity\Activity;
@@ -10,6 +19,11 @@ use Doctrine\ORM\EntityManagerInterface;
 use libphonenumber\PhoneNumber;
 use Symfony\Bundle\FrameworkBundle\Test\KernelTestCase;
 
+/**
+ * @internal
+ *
+ * @coversNothing
+ */
 class ThirdpartyMergeServiceTest extends KernelTestCase
 {
     private $mergeManager;
@@ -33,7 +47,7 @@ class ThirdpartyMergeServiceTest extends KernelTestCase
         try {
             // Rollback the transaction after each test to ensure no data is persisted
             $this->connection->rollBack();
-        } catch (\Exception $e) {
+        } catch (\Exception) {
             $this->connection->close();
         }
 
@@ -47,7 +61,7 @@ class ThirdpartyMergeServiceTest extends KernelTestCase
         $toKeep->setName('Thirdparty ToKeep');
         $toKeep->setEmail('keep@example.com');
 
-        $toDelete = new Thirdparty();
+        $toDelete = new ThirdParty();
         $toDelete->setName('Thirdparty ToDelete'); // This should be ignored
         $toDelete->setTelephone(new PhoneNumber('123456789'));
 
@@ -69,7 +83,7 @@ class ThirdpartyMergeServiceTest extends KernelTestCase
         $this->em->clear();
 
         // Verify data was merged correctly
-        $mergedThirdparty = $this->em->getRepository(Thirdparty::class)->find($toKeep->getId());
+        $mergedThirdparty = $this->em->getRepository(ThirdParty::class)->find($toKeep->getId());
         $this->assertNotNull($mergedThirdparty);
         $this->assertEquals('Primary Name', $mergedThirdparty->getName(), 'Name should remain unchanged');
         $this->assertEquals('keep@example.com', $mergedThirdparty->getEmail(), 'Email should remain unchanged');
@@ -82,7 +96,7 @@ class ThirdpartyMergeServiceTest extends KernelTestCase
             'Activity should be linked to the merged Thirdparty'
         );
         $this->assertFalse(
-            $updatedActivity->getThirdParties()->exists(fn($key, $tp) => $tp->getId() === $toDelete->getId()),
+            $updatedActivity->getThirdParties()->exists(fn ($key, $tp) => $tp->getId() === $toDelete->getId()),
             'Activity should no longer reference the deleted Thirdparty'
         );
 
@@ -94,7 +108,7 @@ class ThirdpartyMergeServiceTest extends KernelTestCase
         );
 
         // Ensure the 'toDelete' entity is removed
-        $deletedThirdparty = $this->em->getRepository(Thirdparty::class)->find($toDelete->getId());
+        $deletedThirdparty = $this->em->getRepository(ThirdParty::class)->find($toDelete->getId());
         $this->assertNull($deletedThirdparty, 'The deleted Thirdparty should no longer exist in the database');
     }
 }