> 99.9% of websites on the Internet will only let you create one account for each email address. So if you want to see if an email address has an account, try signing up for a new account with the same email address.

So why not just send an email upon signup with existing email and just show success on signup? I'm guessing email would state that you already have an account there, maybe you need to reset password and email would provide instructions. Using this approach there would be no leakage of information and you can inform the user someone tried signing up with their email.

Edit: This would work only if only email is required for signup, but it would not work with usernames.