Slaver's Exile

Hello, I've been wondering if you can just keep killing Duncan over and over in Slaver's Exile. Like once I beat all the other sub dungeons, then I kill Duncan, do I have to abandon the quest and beat sub dugeons again?
Or do I not have to abandon quest and just beat the other sub dungeons again to kill Duncan?
Or once I beat Duncan, can I just keep re-entering Slaver's Exile to only kill Duncan?
Thank you in advance.

It used to be that only 1 person in your party had to have duncan ready, so there were runners who would have the other dungeons done, then run a team to slavers for duncan and leave. The team would have to do it 1 person short, but the runner could just zone back to town and run another team. They fixed that in an update, so now you can't. Now every person on the team has to have the other dungeons crossed off, and not have duncan crossed off. Might as well take the reward and start over, there's no way to just keep beating duncan. And there's nothing more annoying than getting a pug to duncan and having the door be closed, and nobody will admit its them that isn't ready for it.