Ok so I bet a lot of people already discovered this but this thread is for the people that haven't. To do this you need to have downloaded the code for the bonus weapon not traded for it, and another thing it takes about 15-20 minutes for each copy of the weapon, so this is for people that don't have anything else to do in game.
1.Start a new game on the same profile that you downloaded the bonus on.
2.Go through the game as fast as you like until you discover the sanctuary and have all of the categories for the
weapon room unlocked.
3.You should get a gift that contains the bonus in it.
4.Transfer or trade the bonus to another save game and delete the save game that you got the copy in. (So you don't take up that extra memory space.)
5.Repeat as many times as you like.
It may not seem worth it to do this but if you know someone that will pay big $$$ for one or trade that last legendary weapon you don't have then it's easily worth it.