Why does my newsletter keep sending the same copy our time and time and time again?
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.