A new bug!?

During my gameplay in minecraft SP, i started to notice that some of my mobs in my home (cows, chickens etc) started to duplicate. at first i thought i did something, but then for some reason, my iron golem duplicated making two.. now for some reason, this bug is actually pretty cool for me, and i wanted to know if someone else is having the same problem. i'm not using any mods for this either, just the original game.