Generally speaking once you have resolved situation, either through violence or other means, you expect the player to move on. This creates two or more "core" methods of resolving each circumstance. For an extreme example, it would be unusual for a person to talk someone down from committing suicide only to follow that action by killing them. The idea of "double dipping" as you have mentioned is a real thing and currently very possible, but you can't balance exp around that possibility because most players are not going to do it.
We actually discussed this idea earlier where I suggested removing exp from killing enemies after you resolved a situation peacefully and other people don't like that solution for their own reasons. I don't really care whether it gives exp or not because I will choose not to do it. If other people do, for whatever reason, good for them.
I don't remember what reasons those are, but they're probably bad and should not be encouraged. This is a roleplaying game, not a number maximizing game. If you have the choice between resolving a situation peacefully or violently, those each have consequences.
I vote in favor of setting NPC XP to zero if you make the choice to resolve a situation through peaceful methods and succeed. That is the consequence you get for making a choice to be peaceful. You should not be rewarded for then doing the opposite.
