Dupe bug with droppers

There's a dupe bug, it does not need too much, but gives you an infinite item.

Video explanation: http://www.youtube.com/watch?v=BMYfy-Ujd-I
(its hungarian, but you can see the building)

All you need to do is to build this thingy, with a dropper, a TNT, and a bit of redstone

Like this:
(There's an obsidian behind every non-obsidian block, just to not blow itself up.)
Top view:

(Repeaters: 7*4+2*3)
Front view:

Press the button, get into the dropper, and take out the item when the redstone torch blinks! It gives you back the original, and an infinite item.