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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.
Duncan, SC Profile: Facts, Map & Data. Duncan, South Carolina - Basic Facts. The Town of Duncan had a population of 3,455 as of July 1, 2016. Duncan ranks in the upper quartile for Diversity Index when compared to the other cities, towns and Census Designated Places (CDPs) in South Carolina. See peer rankings below.