I want to get 2 identical Unkempt Harolds for my Gunzerker, and I do not want to Tourge token grind. I have one already, but I want to get another. How would I duplicate with the newest version of PC BL2?
Duping works just like it did on Borderlands 1, you simply have a friend make a multiplayer game, join as a guest, drop the item(s) you want, then force quit the game (alt F4, don't use the menu). If all works as planned, the host will still have those items, and since your game didn't save the state of you not having the item, your character has it too.
The character duping weapons should join as a guest so this goes smoothly. I'm not aware of any way to do this single player on PC or console (though single players on PC can simply use the Gibbed Save editor to do basically the same thing). Do NOT do this with random matchmaking players, do it with friends for obvious reliability reasons.
Since there's no particularly simple way to prevent this, duping has never been (effectively) patched out of the game.
Note if you do this, Steam may complain about cloud saves not matching local saves. Always continue on local saves when doing this, as your cloud save is probably out of date due to the force quit.