In order to dupe items (including weapons) as well as money you need to be in a co-op game with someone else.

Stand next to your mate and start a trade with them. Give them the items, weapons or money that you want to duplicate.

As soon as you've done that, exit to the Xbox Dashboard by pressing the Xbox Guide button (ie do not save!).

When you go back into the game, what will have happened is you will still have the items you were gonna trade (because you didn't save), but the other play you were going to give them to will have them as well!

You can either then get them to give you the items back (like if it's money) or let them keep it.

Do this as many times as you want!