When the subscriptions cron runs it updates a table that stores the last message id in each thread. So, when the subscription cron runs again it will only send out posts that were made after the last time it has run. It's not based of the user's last visit to the thread at all, so no duplicates will be sent <img src="https://www.ubbcentral.com/boards/images/graemlins/wink.gif" alt="" />
