When you make an infinite loop using hoppers and throw items into it they duplicate randomly from what I could tell.
MC-6439
Duplication glitch using the new Hopper block
Resolved
Duplicate
0
Cole Abbeduto
(Unassigned)
Unconfirmed
hopper
Snapshot 13w01b
Retrieved
78th duplicate of MC-5998, please use the search function to see if your bug has already been submitted. Currently over 42% of tickets are being closed as duplicate.