I've made custom registration, user account forms in conjunction with the Custom Fields addon. When testing the forms on the front end I filled out the form for one of my test accounts as the account information was incomplete and because one of the email addresses I entered had already been used it made me go back, but it also wiped all of the data I had already entered and made me start again from scratch. My custom profile form is very long as it is for a corporate university programme and therefore having to start again from scratch will cause our users a great deal of frustration. 

I have not managed to find anything in the forum about this or anything in your meagre documentation.

Please give me some direction.

thank you

Rob