Duncan is a city and county seat of Stephens County, Oklahoma, United States. The population was 23,431 at the 2010 census. Its main claim to fame is as the birthplace of the Halliburton Corporation. Erle P. Halliburton established the New Method Oil Well Cementing Company in 1919. Halliburton maintains seven different complexes in Duncan plus an employee recreational park, but the corporate offices relocated first to Dallas and later to Houston.