Just have it remove and add flags.
Like:
NPC Code:
if (actionplayeronline){
if (!hasread1){
sendpm blah;
set hasread1;
}
}
And then later check for stuff like:
NPC Code:
if (actionplayeronline)
if (hasread1 && !hasread2){
unset hasread1;
sendpm message;
set hasread2;
}
}
But, you might have to unset every old flag every time, if you want to take up less space.
Your method might work, I didn't put much thought into doing it that way though. If you wish, I'll try to help you do that way.