I've a couple of questions regarding stacking multiple area effect spells:
Does Celestial Brilliance spell (BoED p94) stack with itself? That is, if two instances of this spell are cast and an evil outsider is within both areas of effect, does it take 4d6 points of damage or 2d6? Why? (there are plenty of spells like that, e.g. wall of fire, it's just Celestial Brilliance which is easy to stack)
What if it was an emanation, instead of an unspecified area effect?
What if it was a spread?