Yes! Seatrobot checks for duplicates by unique email address.
When you create an invite Seatrobot checks all the email addresses in that invite, plus all other invites already saved and / or launched. If it sees an email duplicated it will remove that contact from the latest invite.
Similarly if a guest has already purchased a ticket to your event or registered via the register page, you will not be able to invite a contact with that same email address to that event.
So why did your guest receive two invites to the same event? The most likely explanation is that this contact appears more than once in your database with different email addresses. If this happens Seatrobot will count both entries as separate contacts.
Note: if your Seatrobot system is linked to another CMS, like Salesforce, for contact management, it's possible that a single contact may end up with two different unique ID's in that system. This can sometimes result in a duplicate invite.