Restarting enrollment for known user should not create a new petition

In case of self-service enrollment for authenticated users, the user login identifier is known early on in the enrollment sequence. If this user already has a pending petition for the same enrollment flow, it would be expected for the user to get a status message on the state of that petition, instead of starting a second, duplicate petition.

There apparently is code to detect duplicate petitions, but as far as I can see this is late in the process: ie, administrators are supposed to flag them and do something. If the user login ID is known, such duplications can be flagged early on and the user moved to the pending petition instead.