Global Trade Station

This trick requires a Pokemon that knows "Thief" and a Ditto. Enter a double battle with a Ditto without an item and a Pokemon holding the item you want to duplicate. Make Ditto transform into the Pokemon holding the item. Switch the Pokemon holding the item with the Pokemon that knows Thief. Use Thief on Ditto, then win the Pokemon battle. The Thief Pokemon will now have a duplicate of the item.

NOT SURE IT WILL WORK