When I was in the camp I decided to change my pet from the zombie pig (there was only one of it) to the goat. When I closed the inventory I noticed I had two goats, so I tried to fix removing and reselecting the goat in the inventory; this time when I closed the inventory the goats were three and so on... Then I tried to change to another pet (the endermite) but nothing changed.

Seems like deselecting the got doesn't remove/replace it. 

I didn't do anything special to trigger this since I had just started the game in single-player from the main menu and entered the camp.