New Account, change Duplicate E-mail error
You can do a couple of things. One, I would advise installing the Username Enumeration Prevention module:

https://www.drupal.org/project/username_enumeration_prevention

This will stop forgot password form abuse.

Secondly, hiding this fact on the registration side is hard. You can't have duplicate email or usernames in Drupal (not easily), and if you get an error on that then it is pretty apparent it is already in use. Giving a vague error here could also confuse actual users who are trying to register.