On what page is this error occurring for you, what is the URL when you see this error? If the user is already registered then the pp_confirm page should just update the user record, if the record does not exist then it should insert a new one. It sounds like your page is trying to insert the user no matter what.
Please post back with this info and any other details that you can think of. If I'm not able to identify the problem I may need to open a ticket for you so we can exchange the files in question. Let me know.