Mobs despawn by going to a nether portal (Muliplayer + Singelplayer)

I was on my Snapshot server whit my friends, And we want to move some cows to my base. but when we look in the nether the were gone. my friend saw the cow. It was there but after one second it was gone. in the console was this text: [13:26:06] [Server thread/WARN]: Tried to add entity Cow with duplicate UUID 476
02acd-311c-430a-bb0b-626324dcd773

Same problem in SingelPlayer