Carry coals to Newcastle
To do something pointless and superfluous.
Newcastle Upon Tyne in England was the UK's first coal exporting port and has been well-known as a coal mining centre since the Middle Ages, although much diminished in that regard in recent years. 'Carrying coal to Newcastle' was an archetypally pointless activity - there being plenty there already. Other countries have similar phrases; in German it's 'taking owls to Athens' (the inhabitants of Athens already being thought to have sufficient wisdom). 'Selling snow to Eskimos' or 'selling sand to Arabs', which in many people's understanding also have the same meaning, are a little different. Those expressions refer to things that are difficult to achieve, i.e. requiring of superb sales skills, rather than being things that are pointless..
Despite the name of the city, Newcastle's castle keep is almost a thousand years old - having replaced an earlier castle in 1178. The association of the city with coal and the phrase itself are also old. In 1606, Thomas Heywood in 'If you know not me, you know no bodie: or, the troubles of Queene Elizabeth' wrote:
"As common as coales from Newcastle."
The explicit link with pointlessness came soon afterwards, in Thomas Fuller's The history of the worthies of England, 1661:
"To carry Coals to Newcastle, that is to do what was done before; or to busy one's self in a needless imployment."