All dungeon quests are repeatable, meaning you can do it all over again even after you did it, just re-enter the area and you can take the quest again.
i'm asking if you can cut corners, and keep killing duncan the black instead of repeating the first four dungeons prior to the door to duncan being unlocked
I'm pretty sure they fixed that a few updates ago, i think you need to kill the 4 bosses before Duncan. I don't see why they would leave that loop hole in, so i'm assuming you will have to repeat the 4 bosses again.
