Instructions
Select the item you wish to duplicate and equip your Ragnarok character with it.
Access the tower by talking to the manager in solo player mode.
Select the menu at the preparation screen and unequip all of your items.
Go back to the preparation screen and click the arrow in the bottom right-hand of the screen to leave.
Let the screen turn black. Re-open the game and check your inventory. Your character should be equipped with the item, and there should be a duplicate of the item in the inventory.