The reason why you may not be able to login (and hence tried to register again) is because you have forgotten your username and password. In this case you have 3 options.
Go to http://www.medentry.edu.au/login and you can find the link to retrieve password.
Email us so that we can retrieve and send your username and reset your password.
Use another email address to register, but you will be unable to obtain a last year's student discount if you do not use the same details as last year.