Way to do it- Get any bow, and some arrows (the amount of arrows equals the amount of items that will be duped)
*i forgot to put in, "aim straight up before you let go of the trigger"
(Letters in parenthesis = xbox 360 key)
1) Equip the bow and arrows.
2) Hold fire (Right Trigger)
3) Access inventory ("B") and let go of fire trigger
4) Click you're equiped arrows twice (A)
5) Drop the item you want to dupe (X)
6) Exit the inventory and there you go, duped items.