This tutorial will show you how to duplicate weapons, items, and money.
This glitch is very simple and can be done in less than a minute.

1) Start a co-op game with one person or more.
2) Stand next to another player, and initiate a trade.
3) Give the other player the objects you want to duplicate.
4) As soon as you do this, quickly leave the game by exiting to the Xbox dashboard. Make sure you did not save.
5) 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.
6) Join the game and get your friend to give you back your weapons/items/money.
7) This can be repeated as many times as desired.

Notes: Make sure to do this with a trusted friend.
Make sure not to do this near an auto-save point.

If you are having any problems, just watch this video: