To do this you need at least one other player. Start up a save and grab a magazine you would like to duplicate. Now have your other player do the same in their own game. Both save the game. Player B joins Player A and drops the magazine and leaves the game. Player A saves the game. Now Player A has two copies of the magazine! You can do this over and over again to get up to twelve of the same type. So, if you have 12 bargaining magazines the price at pawnshops drop 120%! Or you could get 12 driving books and have your SUV indestructible.