Duelinator is a 35G Achievement for Borderlands. It involves entering a duel (by knifing a person you are playing with) and hoping they will knife you back). The aim is to not take damage.
This is easily done with a friend whilst playing co-op. Go to knife your friend. Ask him to knife you back. Then, ask him not to attack you for the duel, and then do the same for him. The achievement will pop up as soon as the duel is won.
It is also very easy to do against a low level player, or even a same level player with a bit of skill.