When creating your hunter, enter the name VIKEIF
When creating your hunter, enter the name SHUICHI
Create two hunters. The first hunter should have an open item slot, and the second hunter should have the item to be duplicated. Be sure to save the second hunter before entering into battle. During battle, make the first hunter attack the second hunter. Then, have the second hunter give up the item you want duplicated. Finish the battle, but do not save the second hunter. Instead, reload him. Now both hunters should have the item in question.
At the start of the game, press L1 + L2, then press and hold R1 + R2. Wait until you hear a tone, then release the buttons. Finally, press Left, Right, L1, L2, Left, R1, R2, Right.