If you hook up a hopper to a chest and drop an item in it, you can dupe the item. You can only do this for a single item. Any more wont dupe. For example, I drop 1 diamond in the hopper. When I look in the chest, there's two. If I add one more diamond in the hopper, there's 3.
EDIT: Sometimes happens more than once
Duplicate of: MC-5998 - Please use the search function to see if your bug has already been submitted.
Currently over 35% of tickets are being closed as duplicate.