carry coals to Newcastle
- (idiomatic) To do something that is unneeded or redundant.
1851, Herman Melville, chapter 81, in Moby-Dick:
- However curious it may seem for an oil-ship to be borrowing oil on the whale-ground, and however much it may invertedly contradict the old proverb about carrying coals to Newcastle, yet sometimes such a thing really happens; and in the present case Captain Derick De Deer did indubitably conduct a lamp-feeder as Flask did declare.
do something unneeded or redundant