Are you testing this locally? or remotely?
Who is the hosting Provider?
Do you know what kind of mail server they are using?
This usually happens when the SMTP server gets choked up on the volume of emails being sent.
you can try increasing the Seconds Between Emails: in the Power Messenger settings panel.
with some SMTP servers, when the email queue gets backed up they end up chocking on the volume and end up sending duplicates to some users. increasing the time between messages being sent to the smtp server should help alleviate the issues with the queue.