This article is from the Final-Fantasy and other Square Soft Games FAQ, by firstname.lastname@example.org (Nick Zitzmann) with numerous contributions by others.
You can duplicate any item that can be equipped by a member of your
Equip the character with whatever you wish to duplicate. Then, go into a
For the character who is carrying the item to be duplicated, wait for
their turn to come up, and select Item. Select the first available blank
spot in the inventory, then push all the way up until you get to change
the items the character is holding. Select the item to be duplicated. By
doing this, you have unequipped whatever your character was holding.
Finish the battle (by winning or running), and re-equip the character.
You should notice that your character is carrying two of the same item.
Unequip the item, and you'll have two of them in your inventory.
(This works in all of the various Super Famicom/NES versions of the
game. Does anyone know if this bug was fixed in the PSX versions or