Hopper clocks will duplicate items.

Hopper clocks made by connecting two hoppers together will occasionally duplicate the item inside when a chunk has been reloaded. I have only confirmed this in multi-player. It has an effect on hoppers placed in creative and survival mode.

Unfortunately, I cannot get a screen shot of this happening, it seems to be somewhat random. My Hopper clocks in the spawn area are unaffected, but areas that are loaded/unloaded will experience this bug. Best way to replicate is to set to hoppers up as seen in the picture and place a tool, weapon, or any item inside. Leave and return to the chunk several times.