When a purchase is made through a system such as Clickbank and the information is sent to OptimizeMember, our system generates a special link which can be used to access the registration form on your site where the user can register their account
This link is special in that once clicked, it will expire to ensure that the link cannot be shared between other people and allow other unauthorized registrations on your site. This error (duplicate data) usually occurs when the link (either through the email or through the redirect) has already been clicked. Once this happens, the link expires and you wont be able to access through the link
If you go to OptimizeMember > Clickbank Buttons > Clickbank Member Registration Links you can generate a new link for the customer to give them access it they were not able to register or you could manually create the account inside the "Add A Member" section of OptimizeMember
If this is happening with every customer then it may be symptomatic of something more serious going on but generally whenever I have seen this issue it's because the user has clicked one of the registration links more than once and it has expired for security - in these cases we recommend contacting our support for further investigation.