The Duplicator is an admin or map maker item, and is made for servers 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, as it would allow for infinite generation of any item.
To retrieve items from the Duplicator, place an item in the slot in its GUI, and then connect wooden pipes and an Engine (or a filter, a timer and pneumatic tubes). Items will constantly pump out as if it were a chest filled of that item.