I have an HTC One M9. I can go to settings and select Acessibility. Then select Notification Reminder -- if you turn it on will will continually remind you (every 30 seconds or so) of any notification that comes in until you view it.
Mine does the same thing - but for Messages (texts) - and will keep notifying me until I acknowledge the text (read).
No clue why but just started happening out of the blue.
Just read the previous posts (had to register to reply) and will try changing the setting.