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Homefacts City Report. Duncan is located in Spartanburg County, SC. The population is 3,173, making Duncan the 8th largest city in Spartanburg County. There are 7 public schools in Duncan with an average Homefacts rating of B+. The total crime rate for Duncan is low, and there are 17 registered sex offenders residing in the city.
Centrally located in Stephens County, Duncan became the county seat after Oklahoma achieved statehood in 1907. As a compromise between residents of the northern and southern parts of the county, the county courthouse was located in the middle of Duncan's Main Street, half of it in the northern part of the city and county and half in the southern.