Seatrobot de-dupes at each email send. So, the following situations will occur:
If you create an email blast and it includes an email address that has already registered, they will not receive the invite
If you include an email address in an email blast, and subsequently independently that contact registers via the registration link rather than ‘accepting’ your invite, they will be able to register successfully and their ‘invite’ will be marked as confirmed in your guest list. If they then also click on the ‘accept’ link in the email they previously received, they will arrive at their confirmation page telling them they are already registered.
If someone who has already accepted an email blast invite and is confirmed also tries to register at the register page, they will see a message telling them they are already registered and to check their email. They will then receive an email confirming they are already registered with a link to their confirmation page.
If someone tries to register on the register page a second time, the same situation as (3) occurs.
You can feel safe sending invites out to contact lists as anyone already registered will not receive it and people cannot be registered twice.