When sending email using Mass Email, emails are being sent multiple times to recipients.
It is possible for someone to receive the email multiple times if their email address is on multiple accounts and the email was sent via Basic Mass Email. To check if the email was truly sent multiple times to the entire query please view your Organization's Journal:
Click My Organization
Click on the Notes from the day the email was sent for details
If you believe a mass email was generated multiple times but you only sent it once, please contact support and provide the Article ID for reference to your issue.