You will only only be able to dupicate items that can be held in the Left Hand or Right Hand. A great way to get loads of Excaliburs to throw. Go into battle. Now select an empty slot in your item list. Now keep pressing UP until you get to the equipment list for the Left and Right Hand. Select the item that you want to duplicate. Now escape from the battle by holding L+R. Open up the menu and go to the character equipment. Re-equip your item. You will notice it has a "2" next to it. De-Equip it and then Re-equip it, to "split it in half."