When you right click on a user, there would be another button that said Fight. It would challenge them to a duel. There would be a message that appears in the corner of the screen, similar to the trade message. If they accept, the two players would be able to attack each other and deal damage. Then, whenever they decided they wanted to be done, they would just press a button in the corner of the screen to end the duel. There could also be a Party Fight with more than two.
Seems like a good idea, lol reminds me of when White Eagle, an admin, was dealing damage to me in Ponydale after I killed all the lvl 50 clouds she had created.