Hi,
I just wanted to revive this thread as I've noticed a problem with my subscription notices recently and it's possibly related to the OP's issue. I've subscribed to two threads with weekly notification, and recently when the notification comes I get three messages (at various times throughout the day) about the same thread. Since the emails are not sent in rapid succession I'm guessing it's either a very odd mail delivery problem, or (more likely) something do do with the subscription checking code.
Since I don't subscribe to many threads this isn't really that annoying to me, but if someone subscribed to many threads I could see how they would consider it spam!