In this case, where you're probing for user names or emails, you don't own the email, so you wouldn't receive the verification yourself, and thus wouldn't know if the account exists.

This is exactly why most password reset emails say "if you didn't request this, please let us know, as someone may be attempting to access your account".

You shouldn't use usernames in that scenario, just emails. After Signup, you just show a general message that a confirmation Email has been sent. If the account already exists, some policy to notify the account owner can be put in place.