To duplicate an item, you're going to need two Neo Geo Pocket Color systems linked together. First, start your game on NGPC #1, make sure that everything is saved, and then link to NGPC #2. Trade the item to NGPC #2, and then turn NGPC #1 off without saving. Start the game on NGPC #1 again, trade the item from NGPC #2 back to NGPC #1, and now you have two of the same item.