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http://forum.civicrm.org/index.php/topic,13834.0.html
We have the mailing cronjob set to run every 5 minutes. We had 2 mailings scheduled to go out at 9:00.
The 9:00 cronjob started the first mailing which took 8 minutes, and then at 9:08 sent the second mailing.
But, a second cronjob ran at 9:05 and saw that the second mailing hadn't been started yet (since it didn't start until 9:08), so it sent it out.