1870 Federal Census shows
Charles Duncan 42 W/M Growcer b. VA abt 1828
Malvina 30 W/F keeping house b. LA
William 15 W/M b. LA Clerk in Store
Living in New Orleans, Ward 3

New Orleans, Louisiana Birth Records Index, 1790-1899
Given: William Lago
Surname: Duncan
Father: Charles
Mother: Malvine Feasel
Sex: M
Color: W
Birth Date: 05 Dec 1855
Volume: 27 Page: 263

1880 United States Federal Census
about William Duncan
Name: William Duncan
Home in 1880: Rigolette, Rapides, Louisiana
Age: 25
Estimated Birth Year: abt 1855
Birthplace: Louisiana
Relation to Head of Household: Something other than a direct relationship (Other)
Father's birthplace: Louisiana
Mother's birthplace: Louisiana
Occupation: Laborer
Marital Status: Single
Race: White
Gender: Male
Household Members: Name Age
James Stuart 21
Sarah Stuart 25
Alice Stuart 7
William Stuart 5
Lafayette Stuart 4
James Stuart 3
Walder Stuart 1
William Duncan 25
Franklin Nugent 15

New Orleans, Louisiana Directories, 1890-1891
Name: William Duncan
Location 2: 455 1/2 Jackson avenue
Occupation: laborer
Year: 1890
City: New Orleans
State: LA

New Orleans, Louisiana Directories, 1890-1891
Name: William Duncan
Location 2: 272 street Thomas
Occupation: bartender
Year: 1891
City: New Orleans
State: LA

1900 United States Federal Census
about William Duncan
Name: William Duncan
Home in 1900: New Orleans Ward 3, Orleans, Louisiana
Age: 45
Estimated Birth Year: abt 1855
Birthplace: Louisiana
Relationship to head-of-house: Head
Race: White
Household Members: Name Age
William Duncan 45 [dob shown as Dec 1854]Laborer
Frederick S Duncan 13 son b. Jan 1887
Mary P Duncan 6 daughter b. dec 1893
Ellen P Duncan 4 daughter b. Aug 1895
William M Duncan 10 son b. Feb 1890

Looks like William married before 1887 and his wife died after 1890. The one listing I find does not seem to match as both were only 19 in 1886 even though his name is William Duncan and hers is Ellen Payne and they are in New Orleans.

1910 United States Federal Census
about William Duncan
Name: William Duncan
Age in 1910: 54
Estimated Birth Year: abt 1856
Birthplace: Louisiana
Relation to Head of House: Head
Father's Birth Place: Ireland
Mother's Birth Place: Ireland
Home in 1910: New Orleans Ward 3, Orleans, Louisiana
Marital Status: Widowed
Race: White
Gender: Male
Neighbors: View others on page
Household Members: Name Age
William Duncan 54 Fruit Salesman
William Duncan Jr 20 Newspaper Distributor
Ellen Duncan 13 at home

Same person? William Lago Duncan????

Louisiana Statewide Death Index, 1900-1949
Name: William L Duncan , Sr.
Death Date: 29 Mar 1937
Estimated Birth Year: 1856
Age: 81 Years
Gender: Male
Race: White or Mexican (White)
Parish: Rapides
Certificate Number: 04264
Volume: 10

Hope this is of some use.

JR