Hi Emil! Couple of things: First, there is no "Kerens County" in Texas. Kerens is in Navarro County.
Second, if I can offer a tip, it's important to look at all sorts of misspellings of the last name. I have found Dewey Alvin and his parent all over...it's just that their last name has been massacred by indexers and census takers (Dunson, Ducan, etc.).
Long and short...Dewey Alvin Duncan's parents were Adam Henry Duncan, b. Dec. 6, 1887 in Tennessee,and his wife, Lela. The family runs back and forth a good deal between Oklahoma and Texas. In 1900 Adam is in Oklahoma Territory (Chicasaw Nation) with his parents, Marion R. and Sarah M (and 2 younger brothers: Thomas, born in Texas, and Burty (?) born in Indian Territory. In 1910 they are in Jefferson Co. Oklahoma. In 1917-18 Dewey Alvin has his WWI Draft registration in Navarro Co., TX (he had rheumatism and looks like a broken hip. It looks like he says he was born in Margolis, Tennessee?). He's married with 3 children. In 1920 he and Lela are in Navarro (under A.H. Ducan), with the other kids and little Dewey...the handwriting is awful. In 1930 they are back in Jefferson Co. OK (under Dunson) with 11 year old Dewey and siblings.
I haven't given you all the citations, details, etc., because there is SO much. And you have some cousin who has done a ton of work on Ancestry. I would recommend, if you don't have an Ancestry subscription, to go to a library or LDS Family History Center that does have a subscription and check out the cousin's work. It is well cited.