Hiii

If on filling your email Id, it shows the email is already registered this means you have used that email to register for same exam before. I can suggest you that use your email id and then in place of entering a password, try the FORGOT password option. If your email would be registered already, then you will surely get an email for resetting your password. If this will not work, then simply creat a new email ID for yourself and start a fresh registration.

Hope this helps.
Feel free to revert back for more queries.
Thanks!!