Crossword Genius

Once in a __, almost never (4,4)

Ross

I believe the answer is:

blue moon

(Other definitions for blue moon that I've seen before include "Very rare occurrence" , "Rodgers and Hart song" , "In which once is very seldom" , "Once in one is a rare event" , "Very rarely, if once in one of these" .)

I've seen this clue in The Guardian.
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