AlertSite: Why do some of my monitors run twice?

I have a monitor set to run once every 30 minutes.  However, when it runs, it appears to run twice.  See the attached image.  Is this controlled by some setting?  Am I doing something wrong?

Note that the large time gap in the image is due to me disabling and then enabling alerting.  Otherwise, there would be sets of 2 close-together runs, with each set about 30 minutes apart.