[Notifications] Don't add IDs to be removed multiple times

In case an application happens to issue multiple close commands in the
time between we record the first close and it actually being processed.

libnotificationmanager/abstractnotificationsmodel.cpp

@@ -142,7 +142,9 @@ void AbstractNotificationsModel::Private::onNotificationRemoved(uint removedId,
     // some apps are notorious for closing a bunch of notifications at once
     // causing newer notifications to move up and have a dialogs created for them
     // just to then be discarded causing excess CPU usage
-    pendingRemovals.append(removedId);
+    if (!pendingRemovals.contains(removedId)) {
+        pendingRemovals.append(removedId);
+    }
 
     if (!pendingRemovalTimer.isActive()) {
         pendingRemovalTimer.start();