1. get a friend and find a server were u have some stuff to dupe
2. find each other in game
3. have your friend put the item u wish to dupe in his backpack.
4. however u wish to do this step is up to you, the way i do it is, on the count of three the friend disconnects, 1....2... right as ur about to say three grab the loot out of the backpack and say three(IMPORTANT!!) do not close out of the backpack inventory screen! it has a less chance to work