--- Application.php	2015-08-01 05:14:44.000000000 +0200
+++ Application.php	2015-08-12 00:03:45.563138730 +0200
@@ -878,6 +878,15 @@
                     }
                 } catch (Horde_Icalendar_Exception $e) {
                 }
+
+                /* Clear all existing attributes but special attrs */
+                $existing_attributes = $existing_contact->getAttributes();
+                foreach ($existing_attributes as $attribute => $value ) {
+                    if(substr($attribute, 0, 2) != "__") {
+                        $existing_contact->setValue($attribute, null);
+                    }
+                }
+
                 foreach ($contact as $attribute => $value) {
                     if ($attribute != '__key') {
                         $existing_contact->setValue($attribute, $value);