When items are thrown in a hopper, they often duplicate themselves. It doesn't happen all the time, and seems to duplicate more often the more hoppers the items travel through. As far as I can tell, it doesn't matter what type of item is put through. The items don't seem to duplicate when placing them in the hopper like putting them in a chest. They only duplicate when dropped in the top of the hopper. This happens when you're in creative and survival.