Duplication Glitch with Hoppers

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