This is from the lost notes of Kaseki. This may or may not be added officially. Here is what is written...
Day 175: It makes no sense that the same stupid lady would have her cat be lost like a half million times. For the sake of a mission, sure. But IC plot wise, it would make no sense for two people to be like "YO, I SAVED MR.CUDDLES" and the other people suddenly realize, "OMG MAYNE, I DID TOO!"
And everyone would realize they saved the same stupid cat a million times.
This rule would apply to certain raid type missions. So the only way for this to make sense, is to archive a mission after a certain amount of requests. But I'm not gonna count that shiz. Instead, I'll just do it by pages. When a request of certain ranks reaches a certain amount of pages, it's gonna get archived.
D/C - 5 Pages
B/A - 3 Pages
S/H - 2 Pages
Advanced - 7 Requests... or so.
Ad you know what? I'm gonna sign this thing. Someone could totes sell this later for money.
Lots of unnecessary love,