Question

To whom was John S. Duncan married to in 1809?

Answer 1

From familysearch.org , it looks like he married Hannah. 

John S Duncan

United States Census, 1850

Name	John S Duncan
Event Type	Census
Event Year	1850
Event Place	Giles county, Giles, Virginia, United States
Gender	Male
Age	61
Race	White
Birth Year (Estimated)	1789
Birthplace	Virginia
House Number	414

HOUSEHOLD	ROLE	GENDER	AGE	BIRTHPLACE
John S Duncan		M	61	Virginia
Hannah Duncan		F	56	Virginia
John S Duncan		M	21	Virginia
Charles Duncan		M	18	Virginia
Hannah Duncan		F	16	Virginia
Henry Duncan		M	14	Virginia
Hannah Oliver		F	8	Virginia

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.).