Duncan is a town in Greenlee County, Arizona, United States. According to 2006 Census Bureau estimates, the population of the town is 713. It is part of the Safford Micropolitan Statistical Area. Duncan is at the juncture of the Gila River and the Arizona-New Mexico state border. Duncan lies on both sides of the Gila, although the primary portion of the town and the entire downtown area lie on the south side of the river. Duncan was founded in the mid 19th century, and the land was added to the United States as a part of the Mexican Cession. The town of Duncan has been destroyed twice by flood and once by fire, and the majority of the downtown area is now vacant. The town and area are primarily populated by Mormons, ranchers, miners (especially from the Phelps Dodge copper and silver mines in SE Arizona and SW New Mexico). Surrounding smaller towns such as York, Arizona and Virden, New Mexico use Duncan public works and public schools. Duncan and the surrounding area along the Gila River is world-renowned for Native American artifacts such as arrow heads, pottery, burial sites, cave paintings and other remnants of the Anasazi and other pre-historic cultures. Duncan High School competes in many sports, but is renowned for their tennis teams. U.S. Supreme Court Justice Sandra Day O'Connor was born in El Paso, Texas, but grew up near Duncan on the Lazy B ranch, which straddles the border between Arizona and New Mexico. The Day family ran the ranch for many years until selling it; it continues to be run as a ranch. O'Connor later wrote a book titled Lazy B: Growing up on a Cattle Ranch in the American Southwest about her childhood experiences on the ranch with her brother H. Alan Day.