Duff is a graduate of Greenbrier Military School and Virginia Tech. He attended the University of Maryland and completed dental school and a master's degree in anatomy at West Virginia University. He began practice in Charleston by joining his late father, Dr. Ralph S. Smith, in 1977. Dr. Smith is a member of the American Dental Association, West Virginia Dental Association, Kanawha Valley Dental Society, Academy of Operative Dentistry, International College of Dentists and the American College of Dentists. He is married and has three sons; the eldest is Dr. J. Christopher Smith. He and his wife, Kathy, have four wonderful grandchildren.