With the latest stable version ( 126.96.36.199 ) whenever someone posts a new comment it sends out two email notifications at the same moment.
In addition to that, there's also a small issue in notificators logic. Notifications about a new post in a topic should not be sent to me when the poster is me.
More info in forum thread http://vanillaforums.org/discussion/20833/two-email-notifications-for-new-comment
This is fixed in 2.1