Two encounters, two challenges, two times the XP.
XP isn't something you loot from a monster's corpse. It's what you learn from meeting a challenge and overcoming it. If you beat a monster without killing it, whether it escapes or you leave it unconscious or whatever, you still overcame the challenge and get full XP.
Say the PCs encounter a green dragon, beat her down to low hit points and clearly are capable of destroying her, but she manages to escape. She goes into hiding and uses sending to call her identical twin sister for help. Two hours later, the sister arrives in the area, runs into the PCs, and gets killed. Clearly the PCs get XP for both fights, right? So why should it be any different if the second fight is with the same dragon?
What if they killed the dragon in their first encounter, but it was true resurrected and they had to fight it again? Would you remove XP for the first fight because the dragon didn't stay dead?
XP isn't something you loot from a monster's corpse. It's what you learn from meeting a challenge and overcoming it. If you beat a monster without killing it, whether it escapes or you leave it unconscious or whatever, you still overcame the challenge and get full XP.
Say the PCs encounter a green dragon, beat her down to low hit points and clearly are capable of destroying her, but she manages to escape. She goes into hiding and uses sending to call her identical twin sister for help. Two hours later, the sister arrives in the area, runs into the PCs, and gets killed. Clearly the PCs get XP for both fights, right? So why should it be any different if the second fight is with the same dragon?
What if they killed the dragon in their first encounter, but it was true resurrected and they had to fight it again? Would you remove XP for the first fight because the dragon didn't stay dead?