Ok, this is a bit weird. First time I encountered this was when I entered Angarvunde. There's a quest to be done by speaking to the NPC just inside, which I duly did. However I thought I found it all rather familiar, having to clear 2 other areas within the ruin to unlock the gate to the quest reward, and even the twist at the end of it.
After I did that, I got the 2nd word for the Animal Allegiance shout. I checked back to a much earlier save, and I had only the 1st word unlocked. So I chalked this up to deja vu, or perhaps another dungeon with a similar mechanic.
A few game days later I was walking up the road past Kolskeggr Mine. I met the 2 NPCs who were walking the other way, towards Markarth, talking about being the only survivors of a Forsworn raid. Now I clearly remember doing this quest, where I first met these 2 characters in Left Hand mine. In fact after completing the quest back then I popped back into Kolskeggr a couple of times to check that they were ok. And they were, happily mining away in the mine.
I'm not sure what's going on. Surely such quests are unique and once done, remain done.
Anyone else had a similar experience?

Well, I know I have to be careful about the Red Eagle book, which is a pretty well known bug. Another, very recent recurring quest is the fact that the jarl of Winterhold asked me AGAIN to retreive the Helm of Winterhold. I was very reluctant to fetch the helm the first time, and I said FU outright this time. Good thing it's one of those you can still reject. I am not doing quests twice, certainly not for idiotic npc's.

For me Potema's magic glowy stuff in Wolfskull cave returned, though the ritual leader was still dead and nothing was actually happening.