Fix massive action add multiple dropdown (#1004)

* Fix multiple dropdown add massive action

* Update Changelog

Author: Lainow

CHANGELOG.md

@@ -9,6 +9,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ### Fixed
 
+- Fix  `massive action` for adding value to `multiple` dropdown fields
 - Fix for the `COALESCE` part of the `SQL query` for `multiple` dropdowns
 - Fix `search option` for `multiple` dropdown
 - Fix display from Planning view

inc/container.class.php

@@ -1242,7 +1242,8 @@ public function updateFieldsValues($data, $itemtype, $massiveaction = false)
                     // Add new values to existing ones
                     $existing_values = json_decode($obj->fields[$field_name] ?? '[]', true);
                     $new_values      = is_array($data[$field_name]) ? $data[$field_name] : [$data[$field_name]];
-                    $data[$field_name] = json_encode(array_unique(array_merge($existing_values, $new_values)));
+                    $data[$field_name] = json_encode(array_values(array_unique(array_merge($existing_values, $new_values))));
+
                 } else {
                     $data[$field_name] = json_encode($data[$field_name]);
                 }