If you log out while sitting on a horse, it will be duplicated when you log back in. Maybe it's possible to do the same thing with items if you put them in a donkey chest but I don't know. I have only tryed this out on a server.
MC-15553
Dublication
Resolved
Duplicate
0
John
(Unassigned)
Unconfirmed
mob
Snapshot 13w19a
Retrieved
Duplicate of MC-13737, please use the search function to see if your bug has already been submitted. Currently over 55% of tickets are being closed as duplicate.