MDL-78650 mod_assign: Improve group submission action event handling

Don't return an event action when a group submission has already been submitted
by another student.

mod/assign/lib.php

@@ -1584,6 +1584,14 @@ function mod_assign_core_calendar_provide_event_action(calendar_event $event,
             return null;
         }
 
+        $instance = $assign->get_instance();
+        if ($instance->teamsubmission && !$instance->requireallteammemberssubmit) {
+            $groupsubmission = $assign->get_group_submission($userid, 0, false);
+            if ($groupsubmission && $groupsubmission->status === ASSIGN_SUBMISSION_STATUS_SUBMITTED) {
+                return null;
+            }
+        }
+
         $participant = $assign->get_participant($userid);
 
         if (!$participant) {