Only works on items that you can throw and then pick up again. Throw an item (melee weapons, throwing knives), then while the item is still in the air, hit select and go to your inventory. Unequip the item that you threw, then leave the inventory menu and pick up the item. You should now have 2 identical copies of the item. Does not work for guns (they can't be thrown), or bombs/molotov cocktails (they explode on contact and as such cannot be picked back up). Also, if you are using Logan and have his boomerang skill, if the skill procs while you are attempting to do this the duplication will fail, because there is nothing for you to pick up.