DC Generator armature winding

tensorbundle

I've messed up the terms 'duplex' and 'double-layer' armature winding. Do the terms mean the same? I meant do they make m=2 , where m is the multiplicity/multiplexity of coil.
let's suppose a 2-layer lap wound dc generator. What will be the value of m?
the value of m is important because back pitch Yb and Yf are related as Yb= Yf + 2m in progressive lap winding. Kindly help me to clear my confusion. Thanks