Update Webform and not Add if there is an existing value in one of the fields
this would be heavy on resources since it needs to search all 'submissions', searching for the user. An alternative idea:


use Rules to create a 'submission' at new user registration, setting the user as author.
use Node Limit to limit each user to one of that type
use Rules to check when trying to create a new 'submission' (url contains...) and redirect to the existing 'submission' page for that user