messageTray: don't show the same notification in both locations

If a notification is already being viewed from the summary area, and
it re-emits 'notify', don't show it a second time in the notification
area.

https://bugzilla.gnome.org/show_bug.cgi?id=614975

js/ui/messageTray.js

@@ -596,6 +596,9 @@ MessageTray.prototype = {
     },
 
     _onNotify: function(source, notification) {
+        if (notification == this._summaryNotification)
+            return;
+
         if (this._getNotification(notification.id, source) == null) {
             notification.connect('destroy',
                                  Lang.bind(this, this.removeNotification));