Carry coals to Newcastle

from The Collaborative International Dictionary of English v.0.48

Newcastle \New"cast`le\, prop. n.
   A town in England.
   [PJC]

   {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.
      [PJC]