I found out you can duplicate items with multiple hoppers, and am currently in progress of building a giant diamond castle using it.
To reproduce:
http://imgur.com/a/3mKZ4
MC-6075
Duplication glitch with hoppers
Resolved
Duplicate
0
Blake Thomas
(Unassigned)
Unconfirmed
duplication, item
Snapshot 13w01b
Retrieved
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.