Start a co-op game with at least one other player. Stand next to another player, and initiate a trade. Give them the desired weapons/items/money you would like to duplicate. Then, immediately quit the game without saving by exiting to the dashboard using the PS button. Since the game did not save after you traded the weapons/items/money, you will still have them, and the other player you traded them to will also have them. The other player can then give you weapons/items/money while using the same method. Repeat this as many times as desired. Note: Do not trade items near an auto save point to prevent the game from accidentally saving while trading the items.