Android: How to resume an App from a Notification?

you need to set your flags

 notification.flags = Notification.FLAG_ONGOING_EVENT | Notification.FLAG_NO_CLEAR;   
 notificationIntent.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP | Intent.FLAG_ACTIVITY_SINGLE_TOP);

Also, if you never ever want there to be a duplicate activity give it this attribute in the manifest

android:launchMode="singleTask"