You can perform this cheat anywhere in the game, but it works best if you put it to use near a checkpoint and bench. You'll need to be playing with a friend, preferably one who is equipped with a headset so that you can communicate as needed. Start by giving your friend the items you wish to duplicate. After the trade is completed, exit out to the main menu and then immediately have the friend who received your loot save and quit from the pause menu. The trick only works if you follow that precise order, and quickly. If you did it properly, both players should now have the desired items in their inventory.