If the participant only sent a message or two to one contact but not the other, then only one record would display email statistics. If both records were selected as recipients for a message, then the system performs other functions.
Within the Luminate Online product, there are a number of checks in place to prevent an email or other message from being sent out to the same email address multiple times. The system is very effective at cutting down on potential spam. Check out this general article for more information: When setting up an email targeting groups, if there are constituents who are members of both groups, do they receive the email twice?
Luminate Online recognizes the potential duplicates in a participant's address book and acts accordingly when emails are sent out, but it does not take action to merge or delete contact records in the address book without the participant's consent.