What are you trying to duplicate? Scenes, or objects inside the scene?
I've just duplicated one of my objects more than three times and it seemed to work ok.
I've also tried duplicating one of my scenes (file) multiple times and it seemed to work fine too.
One thing that happened to me is that, inside the scene, when trying to duplicate the bust, it reached a moment when the app could not do it because the iPad didn't have enough memory left, could it be this case?