Server log crowded with spam messages...

On my survival server that I play on with my friends, but it bothers me that every time we play the server log gets filled by "[22:21:02] [Server thread/WARN]: Keeping entity Pig that already exists with UUID 9cd1fa98-c4d8-4e36-a59e-55d092773a65"

Is there anyway to fix this? I hope their is, because we don't even have a pig farm or any pigs in hundreds of blocks of us, I've even tried using MCEdit to find any pigs, but just I found 1 pig. I've also tried doing /kill @e[type=!Player,!ArmorStand,!EntityHorse,!MinecartRideable,!Minecart] and still nothing...