Start a co-op game with at least one other player. Drop the item(s) and/or 
weapon(s) you want to duplicate. Then, leave the game through a non-saving 
method, such as signing out of your profile or resetting the system. Since the 
game did not save after you dropped the item(s) and/or weapon(s), you will still 
have them, and the other player(s) who picked them up will also have them.