While I was on multi-player I logged off but on my donkey. When I logged back on I was on my horse but there was also another donkey there (a duplicate) with the same things in its inventory as well. This also happens in single player. Just get on the horse and log off or quit, then log back on or enter the single player game and the other horse entity will be there.
Everything is duplicated as well including the name of the horses and the armour.
Don't ask how I found this! Just logged off on my horse and it happened. Enjoy! (MCN: Ch4osNewb / Howardss)