"DB error: already exists" when creating a Drupal user who already exists as a Contact in CiviCRM
How are you trying to create these Drupal user accounts? We have used User Import module : https://www.drupal.org/project/user_import - for this and it works great. It's really neat in that you can export a Group of Contacts from your CiviCRM -> batch CSV upload them into Drupal using user_import module + create AccountNames (e.g. FirstNameLastName format) and/or dispatch links to everyone as you create their accounts. 

For details please see here: https://wiki.civicrm.org/confluence/display/CRMDOC/Creating+a+Drupal+user+for+every+CiviCRM+contact