Now back in BL 2 you could start a duel, add the items you want duped, and then that person drops all the items then leaves the duel. The other person had items show up in there invintory and the dropper goes back and picked them up off the ground.
Sadly for those who loved duping 2K fixed this. (it was kind of gamebreaking.)

The following method worked on BL2 also, but still works in Borderlands the Pre-Sequel.

The host just has to pick up the duped items, not hard.

*Windows
The guest(s) drop whatever items you want duped. (Make sure no one is doing anything that might cause an auto-save.)
The guest(s) then press ALT-F4 at the same time. (This forces windows to force-close the current program without saving.)
Then they log back into the game.
They still have the items they dropped and the host gets all the items left on the ground. (provided they didn't get an auto-save during the dropping part.)

*Consoles
This works the same way, but instead of Alt-F4 you hit your dashboard button.

If you got an auto-save, have the host give the items back, SAVE and quit. Then rejoin the game and try duping again. (After yelling at the fool who caused the auto-save ;) )

Happy duping!