The Duplicator is an admin or map maker item, and is made for servers and or singleplayer adventure maps where treasure chests have to be constantly filled, or for supplying items to server members (i.e. supplying shovels to a spleef game.) There is no crafting recipe, for it would be highly overpowered.
To retrieve items from the Duplicator, place an item in the slot it its GUI, and then connect wooden pipes and an Engine. Items will constantly pump out as if it was a chest filled of that item.
The duplicator can crash a game and break servers (tested in SMP) if used with some mod items.