In D6 using Poormanscron can cause simplenews to send multiple copies of the same email to the same recipient. Therefore I strongly advise against using Poormanscron.. Use cron(tab) and cron.php instead. See this issue. In D7 we use locking which should prevent sending multiple copies. However you can expect much more timing accuracy and reproducibility when using a real cron. If you want support, please setup cron first.