I have a CiviCRM database with lots of contacts some of whom are now going to start creating Drupal accounts. I'm testing the process and have discovered that when I create Drupal user with an email address matching a Civi contact, it creates duplicate contacts instead of merging.
The Unsupervised Individual Email rule didn't seem to be working and, because it's 'reserved', I couldn't see its settings, so I created my own 'email only' Unsupervised rule with Email set to weight 10 and threshold set to 10. Still creating duplicate contacts.
The process of creating the user doesn't seem to affect the issue. That is, I can 'add user' as an admin or create a user as anonymous and Civi reacts the same way. No deduping.