Make sure you have the item you want to duplicate with you. Enter a battle and select the item option of the person with the item you wish to duplicate. Then, scroll down to a blank space on the item list and press A to place a glove there. Scroll back up to the equipped items, then press A on the one you'll be duplicating, then press B to leave the screen. Next, hold the L and R buttons to run from the battle. Press X to see the subscreen, select Equip and choose the character who has the item. Press A on the empty spot the item used to occupy, then press A on the actual item that's now in the items list. You will now see that your character has two of the selected item equipped.
Submitted by: AndyDark