No. There are several steps we take to avoid sending the same email to the same contact from different Outbound Engine subscribers.
When Outbound Engine accounts are created, the contact list is compared against our existing database and duplicates are not imported into the system.
Our email delivery system will not allow the same email to be sent to an email address more than once. This is true even if they are owned by different subscribers.
We use a first-come, first-served rule for our email service. If an email address is already assigned to a subscriber, it cannot be assigned to a new subscriber until that subscriber cancels their service.