Got this from companion leader. Already been to grove and cleared it. The target is dead and cannot be killed again. Bugged quest?

Wait for the zombie apocalypse.

wait for it to repopulate?

Probably bugged/glitched...I do not clear any dungeons/areas untill I have a quest there...I still cleared areas during for certain quests and then later get a second quest sending me back to the same dungeon/cave/area to collect a specific item. I would already have the item in my inventory but couldn't use it to complete the quest, couldnt actually retrieve it for where it was supposed to be since i already had it in my inventory......basicaly you now have a broken quest....don't let it bother you too much because you WILL have more of them...Bethesda will try to fix these problems but since there is no additional money to be made by fixing these problems we shall see !!!!!

I know when I'v ebeen sent back to a dungeon (like a bandit cave) i get there and the how dungeon has reset again.

This seriously bothers me. Explore we are told. Enjoy the game in a non-linear fashion. Except I'd we do that we break it. Wtf?