Thanks for posting. Although you can have duplicates of the same contact in different lists if you send to multiple lists at once the person would only get one copy of the email. Did you send to multiple lists when this happened or copy and re-send the email to all different lists?
It is possible that an address would get more than one copy if you have the contact in the account with a piece of different information. For example:
John Smith email@example.com
Jane Smith firstname.lastname@example.org
Jane and John would get individual copies of the email (resulting in two sends to this address).
For your privacy, can you email us your address so we can look into why you didn't get the email? social_support(at)constantcontact(dot)com. Is it possible that the emails bounced?