Get into a battle, then have the character who has the weapon to be duplicated select the "Item" command. Select a black space in the item list, then select the weapon to be duplicated. Exit the item list and have your character run away. Then, go to the menu screen and equip the weapon you chose to be duplicated. It should say that you have two of that weapon. Unequip and equip the weapon once more to add an extra weapon to the item list.