We did have an issue with one of our mail servers that caused a queue of messages to back up. When we identified the problem, we decided tha it was better to deliver them late then to not deliver them at all. We are sorry for the inconvenience.
The emails that were backed up in the queue were ones that did not send due to a failed process. There should not have been any that were duplicates. Is it possible that they just appeared as duplicates because they were both notifications about the same thread? These emails contain almost the exact same content, and since they just link to the last page of a thread through a paramater in the query string, they even link to the same place in the thread. The key difference is the timestamp.
Once again, I'd like to appologize for the inconvenience this has caused some users.