Anecdotally, I have at least 3 GitHub profiles
Even for confirmed email addresses, Scratch is super lax in terms of what they'll accept. My company (a childrens' coding enrichment program) probably has upwards of a thousand Scratch accounts attached to a single gmail via "+" aliases, and Scratch doesn't care at all.
EDIT: additional, not "duplicate", which would imply any of the personal data was similar - it was of course fictional.