Why does my newsletter keep sending the same copy our time and time and time again? When I send a Simple News newsletter out in Drupal 6, subscribers get as many as ten copies of the same newsletter. The only way I can stop it is by deleting the page. This problem did not happen in Drupal 5 and only started when upgraded to 6. Any ideas?.
The following issue in Simplenews Issue queue Issue #351379: Keeps sending newsletter provides a few troubleshooting options. It is usually created by an invalid email address (that has a dot at the end for example: email@hotmail.com.) 

If you have an existing Newsletter that is still sending emails you can stop it in the simplenews_newsletter table, set the s_status to 2 (sent) and empty the simplenews_mail_spool table.