Yesterday I crossed something on twitter that I can't yet find again, but the context was participatory platforms and the assertion was that assuming "one email = one person" is problematic, it's a barrier to having family members participate.
I can't remember more; for the moment I'm putting an issue as a tickler so the question is validated and addressed, or determined to be not an issue.
Email is also GDPR-sensitive data, and it's also an attribute that changes. We are already wired to allow IDs or emails...perhaps we can consider backing off using emails as identifiers in our service?