bring coals to Newcastle

To do something redundant, frivolous, or unnecessary. Newcastle was once a major coal supplier. We definitely don't need to bring any toys when we go over their house—they have so many that bringing more would be like bringing coals to Newcastle. I thought a bottle of wine would be a nice gift, but when I saw their liquor cabinet I could tell that I had brought coals to Newcastle. I don't want to bring coals to Newcastle, so you tell me what I should bring to the party.

coals to Newcastle, to carry/bring

To do something that is unnecessary or superfluous. The Newcastle referred to is the city of Newcastle-upon-Tyne, a seaport in northeastern England that was given a charter to mine coal by Henry III in 1239 and became a major coal center. By the seventeenth century this metaphor for bringing an unneeded thing was current, and it remained so in all English-speaking countries. There were (and are) equivalents in numerous languages. In French it is to carry water to a river.