If you already registered before for any course, you don't need to register again.
First find the new course you want to join, then click on the "Book Now" option from top right side of page. Then, on the top of registration form, click on "If you are present student, click here to login".
After you login, you will not need to register again. A new course will be added your profile - the same profile of which you registered.
Just accept to want to join to the new course, and then click on Pay and select one of payment solution and pay for course to add to your profile.