Mobs in minecraft are duplicating for no reason. e.g. I build an Iron Golem, walk away, and when I come back, three more have just randomly appeared. The clones even have the same name as the entity that's been duplicated. As far as I know, this doesn't happen with tamed pets like cats, dogs, and parrots. It's a pretty weird glitch, and I hope it gets patched in the next update.