It's a lot more complicated than that. In open PvE, which is meant to be cooperative, boons are shared, fields can be applied, banners can be dropped, etc., all of which boost a player. How does Anet turn that off just for a duel? And what about duels at an event? Again, Anet separated this stuff for a reason. Edit: I'm referring to boosts from bystanders. Don't think it's nearly as complicated as you think. After duel begins restrict players from receiving benefits from anyone they aren't in party with, or anyone at all. While dueling your opponent doesn't count as a party member so that shouldn't be an issue either.