Things You'll Need
Instructions
Equip the W-Item materia on the character who has the item you wish to duplicate.
Enter a battle by walking around on the world map.
Select the W-Item entry on the menu screen during the battle.
Choose an unimportant item, then use it on any of your characters. For your second W-Item selection, press the Circle button to select the item you wish to duplicate, then press the X button to cancel your selection. This will increase your inventory quantity of the item by one.
Continue alternating the Circle and X buttons until you have accumulated the desired number of items.