Duncombe is a city in Webster County, Iowa, United States. The population was 381 at the time of the 2020 census. Duncombe is na…Duncombe is a city in Webster County, Iowa, United States. The population was 381 at the time of the 2020 census. Duncombe is named for Hon. J. F. Duncombe. Duncombe is located at 42°28′12″N 93°59′34″W / 42.47000°N 93.99278°W.