"There may also be other changes or replies, but you will not receive any more notifications until you visit the forum again."
I just test edited a subscribed page and got the correct edit notification. It's possible that your server just accidentally sent out two copies of the same email. I have gotten duplicate notifications from sites like vbulletin.org in the past. Since vBulletin's behavior is only to send notifications for the same page the FIRST time a change is made, if you made the comment first, you might not have ever received an edit notification - once you visit the forum again, this counter is reset.