Instructions
Start playing the game. Collect some items. Save and then duplicate the save.
Determine which item you'd like to duplicate. Make sure the item is stored in your "Items" list.
During "walkabout" mode, open the menu. Select "Items" from the menu. Put the item you would like to duplicate into the top-left corner of the item list.
Enter a battle. While battling, select the "Item" command for any character.
Hover the cursor over the item you wish to duplicate. Press "Up" and "A" at the same time.
The screen flashes briefly to the "Weapon and Shield" list. After you see the "Weapon and Shield" list, hit the "B" button to complete the item duplication.
Go to your inventory. If the duplication worked, there's one more of the item that you duplicated in your inventory than you had previously.