First you need to obtain 3 or more of the item you wish to duplicate and place them in your junk. Find any merchant (preferably one you haven't sold to or if you have by back all items in his/her junk tab to speed up the cheat) and sell one of the item to be duplicated and highlight the 2 or more in your bag.

Now to the tricky bit, you need to press Y, then A straight afterwards.

If done correctly the items should be gone from your junk, but it will still ask you how many you want to sell if not try again (this trick works better with two fingers, one on Y and one on A so you can press them almost simultaneously)