Website questionnaire with email and password log-in

I need to create a questionnaire for an existing website for which users will have to log in with their email address and password.

The first stage will be where the client registers with email and pays a fee to access the questionnaire. (I am not sure if the password should be user generated or if we should create one for them once they have paid- this will be up to you to decide the best and easiest option)
We will then receive the results of the completed questionnaire in an easy to manage and digest format.
After sometime, the client will then need to access the same questionnaire again- using their log-in details so that we can see the difference in their answers from the first questionnaire.
We need to ensure that an email address cannot be registered twice.