From familysearch.org , it looks like he married Hannah.
John S Duncan
United States Census, 1850
||John S Duncan|
||Giles county, Giles, Virginia, United States|
|Birth Year (Estimated)
CITING THIS RECORD
"United States Census, 1850," database with images, FamilySearch(https://familysearch.org/ark:/61903/1:1:M8D9-5YP : accessed 21 May 2016), John S Duncan, Giles county, Giles, Virginia, United States; citing family 397, NARA microfilm publication M432 (Washington, D.C.: National Archives and Records Administration, n.d.).