Prevent The User To Use The "previous Button" On The Navigator To Avoid Data Duplication

I have a page register.aspx that a user can use to register on my website. Once the registration process is completed, I redirect the user to thankyou.aspx where he will get a code to activate his account.

My problem is this: from the thankyou.aspx page, if the user click the "previous button" on the navigator, he can still have access to the information he had previously submitted and might cause data duplication . I don't want this to happen. I want in that case to redirect him back to thankyou.aspx or another page which will be different than the register.aspx.

My questions are:

1. how can I prevent the user to use the "previous button" on the navigator to avoid data duplication?
2. in case question 1 cannot be done, how can I know in register.aspx that the user comes from thankyou.aspx (which should not normally happen) so that I can redirect him to another page?

My approch was before redirecting to thankyou.aspx, I concatenated session.sessionID and another data to be retrieved on thankyou page. And on thankyou.aspx, I destroy the session after the page was loaded by using Session.Contents.RemoveAll(). And here, nothing happens. The session is still on when using the previous button.