When I returned to Tenpines Bluff setllement to add some defence one of the setllers gave me quest I already cmpleted long time ago - clear Corvega assembly plant from raiders.
Do minor/settlement quests repeat over time or it's just a glitch?

I believe they might be some repeats since there are only so many places. I have not gotten any but then I really have not done many quests and I love to restart a lot.

The place they give you is a random location that isn't already CLEARED. For instance, I got tasked to clear Walden Pond of raiders when I visited Tenpines Bluff because I'd already cleared the Corvega plant.

that little tidbit is from Skyrim in which locations do not respawn. Locations do respawn in FO4 and I know for sure becuase i have cleared out the Corvega factory 3 times. once on my own and twice upone settlemnt request.

Same here. I already cleared Corvega assembly plant - it's one of the first Minutemen quests. Still the "CLEARED" marker on my map disappeared from there recently. And now the settler gave me almost the same quest except for dialogue lines.