By default it should reconcile against the email address they provide, so if the profile already exists it will just update that profile rather than create a new one.
Unfortunately there is no way you can validate the data though, so users will be directed to the same confirmation page, regardless of whether they are a new profile, or if they already exist.
By default it should reconcile against the email address they provide, so if the profile already exists it will just update that profile rather than create a new one.
Unfortunately there is no way you can validate the data though, so users will be directed to the same confirmation page, regardless of whether they are a new profile, or if they already exist.