Originally Posted by
PrincessCupOButter
The way to tell if it's an old request is if it says it is ___days old. When you get a request that says it was sent 2 days ago, that means the player sent it two days ago and reached their limit of 20 one day ago. When they roll over (they don't always and there doesn't seem to be any rhyme or reason to how this works) the next group of 20 neighbors gets the two day old requests. It's usually no big deal, but in this case, when there is a second neighbor request, accepting an old request can cause trouble with the second request. That is why players were advising on here to not send too many requests on day 7, to avoid the rollover.
As for why there are two neighbor requests in the same event, and why there is a request on the last day, I believe the fact that so many players have been forced to complete it with gems explains the reason for that.