I received a quest from the Institute to clear out Sunshine Tidings, but I already did... I already have a settlement set up there, so no matter if I reload the game the zombies will still be dead. Maybe if I completely destroy my 2nd biggest settlement (20 settlers) they'll come back?

When I get quests for that I can usually just return to the quest giver and tell them job's done! Doesn't work here?

Ya same as above. When they give me the quest I just say it's already done and they pay me. Maybe it's bc you have a settlement there. I haven't ran into that yet

heh I just ignore them

So this is FO4's radiant quest I presume?