What are the major differences, if any, between a build designed for Overland PvP and a build designed for dueling?
Your builds in open world PvP will be built around damage, sustain and survivability/movement.
Your builds in duels will be built around cheese, cheese and more cheese.
Duelling builds don't have to have as many situational/utility skills to deal with a bigger variety of threats, and a lot of duellers actually modify their setup based on who they're duelling.
It's crap. If you want to duel me, find me in Cyro. And if you have a special "anti-my-build" spec then my teammate will gladly put a knife in your back
Personally, I've used duels to refine my 1v1 skills, try out different rotations and builds, and get to know more about other playstyles. I don't feel like Cyrodiil offers the same controlled environment to really examine 1v1 play and replay fights with the same opponent over and over. Before BGs, dueling was my favorite form of PvP.
I would not try to build specifically for the duel environment or for a particular opponent. I used what I'd use anywhere else and work on improving myself with that.