Item Duplication involving hoppers

When an item is in a hopper clock for an extended period of time (i.e. I had a diamond being passed back and forth between two hoppers of roughly 6 hours of game time) after a certain period of time the item will duplicate, which will both mess up whatever mechanism you are running, and creates the potential to farm an infinite number of any stackable item. (When i check the hoppers a few days later, i had 3 diamonds where i had previously only had 1)