Perhaps there's an easy way out of this - so I'll keep the details down (for now).
- A new user signs in to Wordpress and
- CiviCRM reliably adds this user's email to the contacts.
- The user gets redirected to a page with a CiviCRM profile on it to get more information on the user.
-> The information entered does not populate the new CiviCRM contact, but adds a new one OR if one enters the user's email address it signals that there's a duplicate entry.
Now the documentation says:
For websites that have logged-in users, you may want to allow people to provide additional information as they register for an account on your website. Similarly, when people fill out a profile form you may want to encourage (or force) them to sign up for a user account.
Latter works (in a different use case); but it's the former option that fails - no matter what I try to tweek in the settings.