In the advanced To address settings, try increasing the time between emails in the email limits section.
try setting the time between emails to 2 or 3 seconds
On some email servers, when the smtp server gets overwhelmed with volume, it ends up sending multiple copies of the email.
you may also need to set a lower number for the "Send a maximum Of setting, maybe set it to send a max of 30 emails every 1 minute to prevent bogging down the smtp server.