The Collaborative International Dictionary
Carry coals to Newcastle
Newcastle \New"cast`le\, prop. n. A town in England.
Carry coals to Newcastle to do something utterly superfluous; to do something useless or wasteful; -- from the nearness of Newcastle to the coal-mining district.
Wiktionary
carry coals to newcastle
vb. (context idiomatic English) To do something that is unneeded or redundant.