Stripe does it wrong, so it's wrong everywhere? Nah. That's bullshit. For emails, you can very easily fake a second signup with the same email and ping out an

Stripe does it wrong, so it's wrong everywhere? Nah. That's bullshit. For emails, you can very easily fake a second signup with the same email and ping out an enhanced verification email "Somebody just tried to register with this address but you already have an account". Building the username into that flow is harder. That's why I preference not having separate usernames but some people would argue this is itself a bad thing (removing a factor or somesuch). Either which way, Stripe's approach is broken. That doesn't make the whole idea nonsense.