Set the Ninja "Two Swords" skill on anyone. Equip a shield in their right hand, and the item you want duplicated in their left. Nnow go to a shop that sells the items you want duplicated, choose "BEST FIT" and buy them. Next, go to the formation screen and remove the items. You should now have two of the item you had in the left hand.

For example, to duplicate Excaliber, go to Lionel castle's shop. You'll get a second Excalibur for only 10 gold!