TCB,
From what I understand so far, symmetrical airfoils are most often described by their thickness ratio (max thickness as a % of chord) and the distance from the l.e. where it occurs (also a % of chord).

Looking at the side view of the fuselage with the wings cross-section drawn, I found the root chord was 14 5/8". I also found the max thickness occurred at 4" from l.e. and was 2.244" thick. So you could say it is a symmetrical airfoil with a 15.3% thickness ratio occurring at 27% of chord. I don't know if Platt made it up or grabbed it out of a book yet but I can upload it's dimensions and begin comparing it to other airfoils in the database. Take a look at the Eppler 477. It's a bit thinner than the Duelist but looks interesting.