Ahm, why? You still are winning a fight against an opponent... it could/should have other remafications, but that doesn't change the fact you won a battle.
You can resolve an encounter through combat, or through dialogue. Once it's done, it's resolved, and you should not be allowed to get the XP from resolving one encounter twice.
That why there is such a thing as per enemy XP... a defeated enemy is worth something, always. One could argue that you shouldn't get anything extra for overcoming the actual encounter, but if said enemies are still able to defend themselves/put up a fight, they grant XP when defeated. That's why in most RPGs, fighting already overcome foes like that usually has other, negative ramifications... like an alignment drop, darkside points, etc.
