coals to Newcastle, to carry/bring

To do something that is unneces- sary 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 sev- enteenth century this metaphor for bringing an unneeded thing was cur- rent, 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.