The short answer is... nothing! In order to make sure you do not end up with duplicate subjects, the GotPhoto system will automatically combine subjects with the same data, even if they were registered by different people.
If the same subject has been registered by multiple people, the subject will appear twice on the registration list when your tagging method is set to QR-Tagging — Registration for Picture Day. However, as soon as you turn off registration and switch the tagging method to QR-Tagging — with list of names, the separate subjects will be combined into a single entry on your list of names.
