To duplicate an item (it doesn't matter what it is), you have to be able to kill Chaos very easily(Chaos is the final boss). First, drop the item to be duplicated in or around the bed where you start when you turn the game on, or when you die, ect. Then save the game. Reload the game and pick up your items. Kill Chaos and watch the credits. Once they are done, it will say "Data Saved." Then reload it, and the items will be in your inventory and on the ground!