Prov. to do something unnecessary; to do something that is redundant or duplicative. (Newcastle is an English town from which coal was shipped to other parts of England.) Mr. Smith is so rich he doesn't need any more money. To give him a gift certificate is like carrying coals to Newcastle.
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.