If you put water flowing right to the side of a hopper, then drop items into the water, it will give you 2 of the item once it is put into the hopper.
Here's video for a better understanding: http://www.youtube.com/watch?v=Ldcsx_33rto&feature=youtube_gdata
MC-6151
Duplication of items using hoppers
Resolved
Duplicate
0
Aaron g
(Unassigned)
Unconfirmed
duplication, hoppers, item
Snapshot 13w01b
Retrieved
28th duplicate of MC-5998 , please use the search function to see if your bug has already been submitted. Currently over 40% of tickets are being closed as duplicate.