That can be done once you have access to the Mirage Tower, in the desert. Equip your main character with the gear you want to duplicate and talk with the manager, saying you want to challenge the tower in Single Player mode. In the Preparation Screen, choose Menu Screen and unequip all the items from your character. Then leave to the Preparation Screen again. Once there, hit the arrow in the lower right tip of the screen and you will leave the place. Now, check your character and he STILL will be equiped with the gear you removed and your inventory will also have a copy of them.