GoToWebinar attendee join links are designed to allow for two main scenarios:
The attendee gets disconnected suddenly, due to technical or other conflicts.
The attendee joins from one device (mobile for example), and then wants to switch to a different machine (like a laptop).
For this reason we allow the links to be used several times -- though not unlimited. I'm afraid we don't have the ability to track attendees that are using a link borrowed from a different registrant, but if you see duplicate names in your webinar's attendee list or attendee reports, you could always reach out to clarify who exactly was in session (or dismiss the duplicates if they don't respond).