I.e. Sunburst. There's only 1 roll for catching on fire, correct?

I'd rule that you would make a single check to avoid being set on fire from a single source.

If you are being shot by multiple enemies, though, you would have to make more checks. You can only be on fire once, though; the status effect shouldn't stack.

Armour said:

To help speed up the rolling if the target is hit many different flame and spray attacks you could have them make one test to avoid catching fire but add a - 5% to -10% chance to the roll for each extra flamer attack past the first one, this could work as a way of saying how hard it is to avoid the flames with so much of it in the local area.