You need to have a bow and around 80 arrows
step 1: get ready to fire your arrow but just hold down RT
step 2: whilst holding down RT press the B button
step 3: double click arrows and then drop the item you want to dupe
step 4: bring down menu and pick up duped item (the item number will be timesed by the number of arrows)
my fav items to dupe:
staffs
soul jems
jewelry
food
and scrolls
have fun!