diff --git a/lib/kolab_api_service.php b/lib/kolab_api_service.php
--- a/lib/kolab_api_service.php
+++ b/lib/kolab_api_service.php
@@ -360,7 +360,13 @@
                 if (empty($attribs[$key])) {
                     $result[$key] = $type_attrs['fields'][$key] = $value;
                 } else {
-                    $result[$key] = $attribs[$key] = $value;
+                    if (!empty($type_attrs['auto_form_fields'][$key]['optional']) &&
+                            $type_attrs['auto_form_fields'][$key]['type'] == "list") {
+
+                        $result[$key] = array_unique(array_merge((array)$attribs[$key], (array)$value));
+                    } else {
+                        $result[$key] = $attribs[$key] = $value;
+                    }
                 }
             }
         }