To copy weapons and shields
1) find a battle
2) go to items
3) select an empty space, and replace it with your weapon/shield
4) when you're done with both hands, fight and let another charecter put nothing in hand
5) finish the battle and re-equip the weapons

NOTE:You'll be holding two of that item. To, use only one, didarm and rearm. No you'll have one weapon in hand and the other in your inventory. This is useful if you need extra money.

Secret Item
After the fight with Golbez in King Giott's castle, he'll tell you to go to the tower of Bab-il. Right after he's done talking, use Rydia's warp spell and you will be transported into the crystal room. There will be a dark crystal there that you can take. I'm not sure what this does, but you get an extra dark crystal.