[gnud]

That's a confusing and annoying UI, to be sure - but for these systems, the email adress is not the identifier. The (email,account issuer) pair is the identifier.

So you can have two accounts, say for ([e-mail], Microsoft Account) and ([e-mail], Contoso AD) - and there is no collision and no possible confusion on the system end. All the confusion is on the human end.

And there is a lot of confusion on the human end :)