Too Many Alerts! - multiple emails for the same alert

Just a short post regarding a situation with an alert. 

The problem:  A user was receiving 5 email alerts anytime any changes were made to a calendar.  She sent me copies of the emails - they all had the same date/time stamp and the same content. 

I confirmed (through Site Settings->User Alerts) that she only had one set up.  I also contacted several other users who had the same alert set on the same list but they were only get one email for each change.  And there had been no similar issues reported for alerts on other sites/lists.

I asked the user to delete the alert and add it again.  While I was waiting for her to test and report back, I found some great alert troubleshooting tips here - http://sharepointalert.info/troubleshooting-sharepoint-alerts/ - and even contacted the author (Ryan with Pentalogic.net).  He also suggested that we delete and re-add the alert.

The solution:  Delete the alert and re-add it.  After the user did this and waited for a change to be made to the calendar, she only received one email.  I wish I knew why this corrected the problem.  But, as with many other issues, a simple "re-boot" is often the solution.