Duncan is a city and county seat of Stephens County, Oklahoma, United States. The population was 23,431 at the 2010 census.The Chisholm Trail passed to the east of Duncan prior to the town's founding. An estimated 9,800,000 Longhorn cattle were herded up the trail between Texas and Abilene, Kansas during its existence. After learning that an extension of the Chicago, Rock Island and Pacific Railroad was being built from Kansas to Texas, Scotsman William Duncan brought his wife, parents, and other relatives and created a trading post situated at the intersection of the north-south Chisholm Trail and the east-west military passage between Fort Arbuckle and Fort Sill. The first train arrived on June 27, 1892; that date is considered the official birth date of the town.Many of the city's first buildings were wood-frame, but were replaced by sandstone and brick structures after natural disasters destroyed them. Four fires in 1901 burned down several buildings.