I think there needs to be a way to accomodate people wanting to fly different aircraft.
I know that many people will fly Spitfires. That's ok with me, but maybe I would like to duel in other craft. I wouldn't want to be forced into 5 Spitfire duels because that is what the guy picked first. Of course, I really don't care if the other guys wants to fly nothing but Spits, but for me that would be dull.
Those that have dueled me know that I let you select whatever you like, and ask you what you want me in. If you say you don't care, I pick something that should make the fight interesting for both of us. I would hate for the rules to prohibit this, as I'm interested in 1 vs 1 fights, not necessarily endless Spitfire fights.
I guess what I'm saying is if both sides agree it should be possible to fly whatever they want. If even one side says they want same planes, then it's same planes.