Why is quickbooks sending the same invoice email multiple times to the same person

It is happening on two customers only, I've spoken to the help desk today and they are not sure why it's happening. 

We think the issue is on the email cc and bcc not the customer email, I sometimes get one email but on the customers with issues 2 or 3. 

We've checked some settings so will see what happens next time. I will probably start each invoice from new rather than copy the last ones. I did try that earlier with no luck but will give it another go next time.