unfortunateface

What sp332 said - there wouldn't be any extra steps as you would have to go to your email to complete registration anyhow.

You could have an extra link in your email which lets you update your existing registration details with the ones you've just entered, but that's not going to significantly extend the amount of time you take to register.

I would argue that registration pages should not let someone know if an email is in use - if a duplicate email is used in registration then the site should just send a quick email notifying the user that this email is already in use (with sensible DOS protection and email opt-outs etc.)

Your use-cases miss out one type of hacker/spammer/bad guy - the one who is only looking for you, they know your email, they want to profile you specifically.

Over the top/exaggerated example (with many implicit caviats!)...

Mr Smith has Mrs Smiths email - he uses it to try to register with a Dating Website - but hold on, it's already registered!

Now there could be 3 reasons for this...

1. infidelity

2. It pre-dates the relationship

3. A third party trying to cause trouble by signing Mrs Smith up.

I don't believe a company has the right to decide what data they hold on you is visible to the outside world - without expressly asking you.