Hi you,
When you create an account with email for the first time, a password will be automatically generated and sent via the registration email (you need to set up SMTP to automatically send emails when you sign up)
Once registration is complete, you will have to enter your password to be able to log in. In the case like you said, just enter the email or login id in the registration form, then the system will report an error that the email or id already exists and you will not be able to login anymore.
You can check with any email with these 2 steps:
1. Register an account with that email
2. Log out and enter your email address in the registration again to log in