Li Shenron
Legend
I really do whatever I want in those kind of cases
What if the BBEG you killed is resurrected? No XP the second time?
What if he changes to an undead?
What if you fight it again after 20 years?
What if he had gained levels since last time?
I think it's quite subjective... the point of giving no xp for the second time could be because it was just the continuation of the first, but then does it depend whether you already gave xp during the first time?
For me it depends more on whether the second time was a challenge or not. If the second encounter is identical to the first (meaning that the BBEG is just the same, only fully healed), then the players have perhaps the advantage of knowing very well what the opponent's powers are, hence the challenge may be easier, but still if it's life-threatening and resource-consuming, then some xp are deserved.

What if the BBEG you killed is resurrected? No XP the second time?
What if he changes to an undead?
What if you fight it again after 20 years?
What if he had gained levels since last time?
I think it's quite subjective... the point of giving no xp for the second time could be because it was just the continuation of the first, but then does it depend whether you already gave xp during the first time?
For me it depends more on whether the second time was a challenge or not. If the second encounter is identical to the first (meaning that the BBEG is just the same, only fully healed), then the players have perhaps the advantage of knowing very well what the opponent's powers are, hence the challenge may be easier, but still if it's life-threatening and resource-consuming, then some xp are deserved.