Hopper duplicating every item

Hopper is duplicating every item, as shown there: http://youtu.be/cjefLh7Yn1k
You don't need to place chest under hopper do duplicate. Items are duplicated when thrown on top of hopper (you can see it good at last 20 seconds of the video where i'm duplicating chests).

P.S. Sorry for my English. It's not very well at all.