In some situations hoppers duplicate items (see enclosed screenshots):
The first image shows one slot of a sorting machine. Note that only the
upper hopper is filled with 2 wool and 20 other items to block the other slots. The hopper below and the chest are empty. When I throw at least one additional wool into the hopper the redstone torch will fire and the wool will
be sucked into the hopper below and finally into the chest. However when I throw at least one stack of wool the items will bounce for a while on the hopper before all the items can be sucked in. In this situation often more than one stack will arive in the chest. (Note that the behavior is not very consistent. Sometimes the items will be sucked in very quickly and no items will be duplicated and sometimes the items will bounce around for a while and up to two stacks will be generated.)