In Borderlands 2 for the PC, PS3, and Xbox 360, you can easily exploit the co-op feature to duplicate just about any item without much effort.
First, make sure your game is saved with everything you currently have on you. Make sure to do this well away from an auto-save point in the game though.
Next, start a co-op game with at least one other person, and give them everything that you want to duplicate.
Don’t quit and save obviously, you want to force the game to quit without saving. On the Xbox 360, press the 360 guide button and go to the dashboard. On the PS3, press the PS button and exit to the dashboard. On PC, just force the game to close.
Log back on to the game, and you’ll have all of the stuff you gave away, and your co-op partner will to.
Repeat the process a few times and you’ll have enough money, items, and weapons to fit an army.