So I guess the solution is just to begin to allow accounts to always register multiple emails? Although I guess the issue of multiple accounts is still going to exist if the users don't know the initial (private) email that they signed up with though unless there is a different unique ID that everyone will be able to remember.

I'm curious (and not trolling by asking) what a solution might be since email has been used as a unique account identifier for so long it is hard for my brain to think of another option at the moment.