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.
Related issues
duplicates
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migrated
I have also seen this happen with sheep, and other entities. After killing an entity I relog and another one is there with the drops from the last one in my inventory. Did it with a sheep in a fenced area.