I'm an American living in the Netherlands who is learning Dutch. There's an idiom in Dutch that describes performing a needless/futile activity, "water naar de zee dragen," which literally translates to "carrying water to the sea." My Dutch parents-in-law asked me if there was an English equivalent, but I couldn't think of one.
In doing some searches online, I found that the English translation given for the this idiom is always "carrying coals to Newcastle." This was the first time I'd ever come across the phrase, and subsequent searches revealed that it was indeed of British origin, though one site I found did claim that it was an American phrase. However, neither I nor any of my culturally American friends have ever heard of this phrase.
Is there an American English idiom or phrase that carries the same connotations for carrying out a futile activity?