Prevent duplicate notices within 2 minute window

We found that the code to avoid duplicate email notifications was not
meeting our needs.  Due to an issue similar to bug #2555, pfSense was
repeatedly cycling through our WANs and generating error messages for
each one.  Because each message was different from the previous one, it
was causing new notifications to be sent, resulting in hundreds of
emails per minute.  This code prevents duplicates of a given
notification from being sent within two minutes of the first
notification.