carry/take coals to Newcastle

纽卡斯尔是英国的产煤中心和煤输出港口，运煤去此地显然是“徒劳无功”。

to supply something to a place or person that already has a lot of that particularthing

多此一举;徒劳无功

A:Jane asked me how to solve that problem yesterday.And I gave her the suggestion.

昨天简问我怎么解决那问题。我给了她点建议。

B:Don't do such a thing again. You just carry coals to Newcastle. She already has her own ideas. She just pretended to ask you.

下次别干这种事了。你做的事徒劳无益，她已经有了自己的见解。她只是问问你罢了。