diff --git a/entityreference.module b/entityreference.module
index 425c8aa..70d9bcb 100644
--- a/entityreference.module
+++ b/entityreference.module
@@ -211,7 +211,15 @@ function entityreference_field_validate($entity_type, $entity, $field, $instance
   $ids = array();
   foreach ($items as $delta => $item) {
     if (!entityreference_field_is_empty($item, $field) && $item['target_id'] !== NULL) {
-      $ids[$item['target_id']] = $delta;
+      if (in_array($item['target_id'], array_keys($ids))) { // Duplicate values
+      	$errors[$field['field_name']][$langcode][$delta][] = array(
+          'error' => 'entityreference_duplicate_entity',
+          'message' => t('@label: The referenced entity (@type: @id) is a duplicate.', array('@type' => $field['settings']['target_type'], '@id' => $item['target_id'], '@label' => $instance['label'])),
+      	);
+      }
+      else {
+        $ids[$item['target_id']] = $delta;
+      }
     }
   }