I have found a way to duplicate items in Borderlands 2 and its pretty simple
1. You need atleast one friend online to do this
2. The player receiving the items will host the game
3. Be somewhere away from checkpoints, you don't want it to autosave on accident
4. The player that is giving the items should save before doing this
5. Now that this criteria is met the player that is not the host will drop the weapons or items they want to duplicate and the host can pick them up or leave them on the ground doesn't matter then hit the guide button and go to dashboard
6. Start the game back up and join the hosts game again you should still have all of your items and so should the host
Hope this helps.