How to display multiple notifications in android
just replace your line with this
notificationManager.notify(Unique_Integer_Number, notification);
hope it will help you.
Simple notification_id needs to be changable.
Just create random number for notification_id.
Random random = new Random();
int m = random.nextInt(9999 - 1000) + 1000;
or you can use this method for creating random number as told by tieorange (this will never get repeated):
int m = (int) ((new Date().getTime() / 1000L) % Integer.MAX_VALUE);
and replace this line to add parameter for notification id as to generate random number
notificationManager.notify(m, notification);