Negates PPsh doctrines.
Give them SVT upgrades.
Or give them SVT tied with vet, ie at vet3 they also get 3 svt's (tone down RA bonus if implemented?)
I don't think it negates it, if the non-doc weapon is a long-range weapon. If we give them an SMG of some sort, then they would overlap.
It gives you a choice and you'd go PPSh if you wanted close-combat and you'd go DP if you wanted long-range/medium range.
Currently Volks can go STG or MP40 if they go with Firestorm, and those also perform different functions despite being even more similar than a DP / PPSh comparison.
I'd be fine with SVT upgrades as well, if only so it reduces the focus fire that the DP would provide (ala Grens/Obers).