Weekly digest sent with every cron run after update to Drupal 6.15

On Dec. 23 2009 I updated my D6 installation to 6.15. I have one weekly digest newsletter configured that gets sent out every Monday at 10am. Cron runs every 10 minutes. On Dec. 28. at 10:10 am my site started sending out the current digest, but did not stop after the first batch. The site then continued to send out the digest with ever cron run (until I disabled the module after some less friendly emails from subscribers...). I have not had a chance to investigate further but I'd like to know if someone else experienced this problem. Thanks.