“Taking” or “carrying coals to Newcastle” is an English idiom that means doing something completely unnecessary, pointless or superfluous. Newcastle-upon-Tyne used to be a major coal mining area and the UK’s first coal exporting port, and was first associated with pointless activities in Thomas Fuller’s 1661 The history of the worthies of England: in which he wrote, “To carry Coals to Newcastle, that is to do what was done before; or to busy one’s self in a needless imployment.”
Useless activity can also be compared to selling freezers to Eskimos.
The German equivalent of this is Eulen nach Athen bringen/tragen – to bring/take owls to Athens.
In Dutch they say water naar de zee dragen (to carry water to the sea).
In Biblical Hebrew a pointless activity was bringing straw to Ofarayim (a place known for its grain production).
The Yiddish equivalent of this idiom is tregn shtroy ken mitsroyim (bringing straw to Egypt).
In Russian they say ехать в Тулу со своим самоваром (to take a samovar to Tula) – Tula is a town where samovars were produced.
Brazilians describe pointless activities as raining on a wet place (chover no molhado). Spanish speakers do the same – llover sobre mojado (to rain on a wet place), or, say, compare such activity to ploughing the sea (arar en el mar).