Logging (already registered) user through registration form?

In this scenario, Login and Registration form both have same fields: Email and Password. However both forms are on different pages. By default, Registration form is displayed when user lands up on the home page.

What would be optimal user experience for already registered user, when user tries to login using registration form with valid email and password

  1. Should error message "The email is already registered" be displayed
  2. User should be logged in and taken to the registered account
  3. any other suggestion