It is showing I have already registered whenever I try to register , what is this indication ?

This indicates that your GSTIN has already registered on the electronic bill portal. You have also created your username & password on the electronic bill system. These credentials will allow you to log in to the e-way billing system. Forgot 2 usernames or passwords? Please use the "Forgot Username" or "Forgot Password" facility on the portal to retrieve your username and create a new password.