All useable items (not weapons/armors/magic) can be duped using this method, as mentioned,
1. Open the menu during "walkabout mode" (i.e., not in a battle).
2. Select "Items" and put the item you want duplicated in the top left corner of the item list.
3. Engage in a random battle. During battle, select the item command for any character.
4. Your cursor should be at the very top left of the item box (and on the item you have placed there). Press UP and A at the same time. This should briefly take you to Weapon and Shield lists. If you see this, the code has worked after you hit the B button. Go to your inventory and you will notice that the item quantity has been added by one. You may do this for any item that is stackable.