1
general::
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.
American Heritage Idioms
2
general::
Phrase(s): carry coals to 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.
McGrawhill's American Idioms And Phrasal Verbs