By placing an item in between two hoppers, the items dispensed into a chest are duplicated. Very simple, place two hoppers together, side by side, connected to chests. When thrown an item in the middle, the item will not disappear, but constantly duplicate.
Refer to: http://youtu.be/G7n7LlUTQpg?t=11m21s
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101st (yes, one-hundred-first...) 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.