To add (I would edit my post above but I can't) my friend just told me that the first time it didn't have an error but redirected back to a sign in/sign up area, not a registration confirmation.
This leads me to believe it's not just a simple error and the email in use is indeed a valid output, so the registration should be in the ACP somewhere. If an admin is available to do so, could they check the IPB ACP and see if members registered in the last few days have some discrepancies and/or manually validate them?