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.
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I used search function. I saw few nearly like that, but not the same. I clicked first few links and in any of them there weren't the same problem shown. That is not a duplication of MC-5998 in 100%!!
66th duplicate of MC-5998, please use the search function to see if your bug has already been submitted. Currently over 40% of tickets are being closed as duplicate.