More than once I've been killing a group of mobs (let's say three) and the first one I killed will respawn before I finish off the other two, then after the initial 3 are all dead and I'm working on the freshly respawned one, the other two respawn as well, turning a 3 mob group into 6. I can't think of why it would do that. It should probably NOT do that!
Usually mobs will stay dead for several minutes. It's just huge outliers like this that make it very frustrating to do content sometimes, and there seems to be no way to predict it, since there's never anyone else around when it happens. Maybe the game is trying to respawn an enemy that's still alive and gets stuck trying to respawn it, so it happens immediately when the mob dies?