A duplication glitch with the hopper allows any and all items/blocks to be duplicated. Let the hopper stand alone, or put a chest under it if you choose, then throw the item you want to duplicate on the edge or corner of the hopper. If what you throw is not a stack it will multiply by 2 and all go in the hopper, but if what you throw is a full stack, then that stack will remain on the edge of the hopper and duplication will continue to fill the hopper or chest underneath. Sometimes if you use a full stack to duplicate, a fifth 'ghost' item appears in the fifth slot of the hopper. If you attempt to double left-click, it will not stack/move with the rest of the items, and if you pick it up and attempt to stack it, it will disappear. This 'ghost' item CAN be used in crafting but not with repairing items. This glitch is available both in SSP and SMP.