colloquial idiomatic translation of: "beating a dead horse"?
'Beating a dead horse' implies 'on and on, endlessly debating or citing a same tired subject' (doesn't matter if the subject is settled or not)"

I think it can be described as '沒完沒了' in Chinese. To be specific 'beating a dead horse' is "沒完沒了地重複相同的話題 " (endlessly repeating the same topic)