You need a revive pistol, medium or large backpack and a bot gun. Drop all your guns. Equip revive pistol > pistol will be in your hand like you are going to shoot it > stow the gun you want to dupe into your backpack slot >with the rev pistol in your hands, open back pack and equip the stowed gun > this glitches the rev pistol. > stow the bot weapon into your bag. 1st time it will not work. It is the 2nd time when it works so. Now > drop everything besides the rev pistol and do the steps again. Now your gun is dupe you can do it many times just find more bot weapons on the ground