Isn't it amazing, "the bible" chapter 3 has the answer.

I have a similar question... Will it be the same if the new interrupt comes from the same source? i.e. I read edges on INT1, a short impulse launches my INT1 interrupt and then yet before I issue RETI, another impulse arrives.
Yes, you will go right back into the ISR after one mainline instruction.
What with even more impulses during one interrupt?
How far can you count in one bit?

Erik