Equip the item to be duplicated to Labyrinthia. You must only have one of that item. Enter a Free Battle against a Harpy. Have Labyrinthia create a Replica in front of the party. Wait for a Harpy to use "Steal Item" on the Replica. Have the original Labyrinthia use her copy of the item then complete the battle. You will now have "0%55" (actually 255) of that item. You can now sell that item for easy money. Note: Other items in your inventory may become inaccessible in some menus. To fix this, save the game and reload it after you are done selling.