One thought that always comes to mind when team members get together to register and then report difficulties...When someone concludes online registration, they're automatically signed in to the Event on the premise that they'll now want to access the Participant Center to "...set up and send". If they immediately proceed to sign up another person without first signing out, an "already registered" or "duplicate email address" error will be generated. Lesson learned: Start every registration anew and make sure you're not signed in to the Event.

As to adding participants offline via the EMC, you can avoid the duplicate email edit check by not entering an email address at all; it's not mandatory in offline registrations (as configured in South Atlantic Division; this may vary across Divisions).

/Rick