Do or bring something superfluous or unnecessary, as in Running the sprinkler while it's raining, that's carrying coals to Newcastle. This metaphor was already well known in the mid-1500s, when Newcastle-upon-Tyne had been a major coal-mining center for 400 years. It is heard less often today but is not yet obsolete.

To do something that is obviously superfluous; Newcastle is a city in northeast England where coal is mined: “Karen wanted to give Dad a magazine subscription for his birthday, but I said that would be like carrying coals to Newcastle, since he already has fifteen or twenty subscriptions.”