Newcastle

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.

to take something to a place or a person that has a lot of that thing already Exporting pine to Scandinavia is a bit 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.

Any unnecessary activity. Before the days of railroading, goods and commodities were transported by water. Coal in particular was shipped to port city of Newcastle before being distributed to the rest of England. Therefore, unless you were the captain of a ship laden with coal, carrying that kind of fossil fuel to Newcastle was a waste of your time and energy.