This trick involves farming canned foods and painkillers so you must first activate the 'Heavenly Gift' and 'Kill the Pain' side missions. When these side missions have been activated proceed through the game to the point where you meet Dr. Kessler. Check out the video below to see what you must do next.
Perform the following trick to duplicate a weapon in your inventory. Equip the weapon you want to duplicate, hold the right trigger and then press the left trigger to throw it. When the weapon is in mid air press the 'Back' button to go to your inventory and unequip the weapon you threw. If you now leave the inventory and pick up the weapon that you threw and once again return to your inventory you will notice that the weapon has duplicated.