So, the rule would be, then, that you disregard the piece's key signature when a note is double flatted or sharped?
Yes, but you ask in a way that might suggest you think this is a rule that would make it different from a "normal" # or b sign?If so, then it isn't afaict.You don't regard the key sig for any note that's explicity marked with a #, b or a double-sharp / flat or natural sign.