When someone is being repetitious
So you said.
Stop beating a dead horse.
beating a dead horse = continuing to argue a point that has been won
Stop harping on that subject.
harping on = dwelling on; talking about
You sound like a broken record. (idiomatic)
broken record = a grooved LP album with a scratch that makes the same track repeat endlessly
Must you belabor the point?
All right, already.
We get the point, already.
We heard you, already.