Multiple items
This trick works when only one of the item to be duplicated is present. Enter a battle and perform the following actions:

1. Have the first character (typically Jack) use a healing item, such as a berry or medicine.
2. Have the second character (typically Cecille) also use the same healing item.
3. Have the final character (typically Rudy), exchange the positions of the healing item and the item to be multiplied in the inventory. Then, have Rudy use the healing item.

After winning the battle, the inventory will contain 255 of the item that was multiplied.