Flameskulls have the following traits (Monster Manual, p. 135):
Eldritch Rejuvenation. A flameskull's shattered fragments reform unless they are splashed with holy water or subjected to a dispel magic or remove curse spell. If it can no longer fulfill its intended purpose, the re-formed flameskull is beholden to no one and becomes autonomous.
Rejuvenation. If the flameskull is destroyed, it regains all its hit points in 1 hour unless holy water is sprinkled on its remains or dispel magic or remove curse spell is cast on them.
If you defeat a creature like a flameskull, and wait for an hour until it rises again, then you kill it again, wait another hour, and kill it yet again… would you earn the XP each time you kill it? (Basically making an XP factory.)
Is there any rule which forbids that exploit?
Related question on turning defeated enemies into undead to defeat them again. This question differs from it because the creature is returning by its own natural ability and not a mechanic provided by an adventurer, which might affect whether it counts as “defeated” for the purpose of earning XP.
Best Answer
No
The DMG says:
Until the Flameskull has been defeated you get no XP for it. It is not (necessarily) defeated by being reduced to 0hp; this is an example of what defeat might look like that is not applicable for this particular monster.
If the objective of the PCs is to get past the obstacle the Flameskull represents (e.g. to loot the treasure it’s guarding) then it is defeated when they do that. If their objective is to destroy it, then it’s defeated when the sprinkle its remains with holy water. Whatever the objective is, it can only happen once and the DM decides when that happens.
Now, it’s perfectly reasonable to defeat an enemy more than once if circumstances warrant: for example, they could defeat the Flameskull by getting its treasure and then, after learning of its regeneration ability and how to stop it they could defeat it again with this new objective.