sell snow to the Eskimos
sell snow to the Eskimos {v. phr.}
To sell something to people who already have a large quantity of the same or similar goods.
My Alaskan friend said, "One of the hottest businesses in Alaska is refrigeration. You could say that I, as a refrigerator expert, am selling snow to the Eskimos."
See: CARRY COALS TO NEWCASTLE.
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Source: A Dictionary of American Idioms