MAGETWO-82951: Check attribute unique between same fields in magento commerce [backport] #11622

 - Merge Pull Request magento/magento2#11622 from raumatbel/magento2:FR#CheckAttributeUniqueEE
 - Merged commits:
   1. 737a76b

Author: Oleksii Korshenko

app/code/Magento/Eav/Model/Entity/AbstractEntity.php

@@ -908,7 +908,7 @@ public function checkAttributeUniqueValue(AbstractAttribute $attribute, $object)
                 'value' => trim($value),
             ];
 
-            $entityIdField = $attributeBackend->getEntityIdField();
+            $entityIdField = $object->getResource()->getLinkField();
             $select->from(
                 $attributeBackend->getTable(),
                 $entityIdField