Method: Equip a character with the weapons you wish to duplicate. Make sure you have an empty space in your inventory. In battle, open up the Item menu, and select a blank space. Move up until you can select your weapons, and change it with the one you want to duplicate. Run from battle, and open up the weapon screen. Select the weapon you duplicated, and there will be a "2" next to it. Unequip it and reequip it, and you will have one equipped, and one in the inventory.
Trick: Skip The Sealed Cave.
Method: Once you have defeated Calbrena, and gained control of Rydia, use the Warp spell. You warp back to the crystal room, but the crystal is there. Take it, and when you first go to The Sealed Cave, it will act as if you are just leaving. (Note: not recommended)