Are Email registrations for an event unique for each email?

Good afternoon. I sent out an email campaign to promote an event from CC. People signed up and then forwarded the email they received from CC to others, encouraging them to sign up. As the new email recipients tried to RSVP, the previous email information was submitted again, entries were duplicated, and the new registrant was not captured. Are the email campaign signups unique to each email address sent from the CC platform? I do not recall reading that emails couldn't be forwarded. We did not have this issue using the event campaign option. Thank you!