Alex - The check is across Leads, Contacts, and Accounts. If you are still experiencing the behavior reported in your post, I suggest opening a support ticket so that a member of our customer team can troubleshoot the issue.
Michelle - In that case it would only send one email to that personnel assistant's email address. Which of the two Contacts it is actually sent to (i.e the Contact the dynamic content will pull from and the Sent Email/Email Event records will associate with) depends on which of the two Contact records is returned first by the query used to generate the recipient list. There is no way to control which of the two duplicate Contacts this is.