Why is my NSNotification its observer called multiple times?

In which methods did you register the observers?

Apple recommends that observers should be registered in viewWillAppear: and unregistered in viewWillDissapear:

Are you sure that you don't register the observer twice?

Based on this description, a likely cause is that your viewcontrollers are over-retained and not released when you think they are. This is quite common even with ARC if things are over-retained. So, you think that you have only one instance of a given viewcontroller active, whereas you actually have several live instances, and they all listen to the notifications.

If I was in this situation, I would put a breakpoint in the viewcontroller’s dealloc method and make sure it is deallocated correctly, if that’s the intended design of your app.