I thought sharing this might help somebody else with the same problem.
Testing my form I have run into problems. The scenario is a membership form which logs in the user. I open a new private browsing window in firefox, fill out the form, get the membership and user login then close that window.
I think open up a new private browsing window and fill out the form and get the error:
The CiviCRM "contact create" API returned the error: "DB Error: already exists" when called by function "preSave" on line 155 of wf_crm_webform_postprocess.inc with parameters: "Array ( [contact_type] => Individual [contact_sub_type] => Array ( ) [webform_label] => Contact 1 [first_name] => test [last_name] => test [gender_id] => 2 [custom_81] => 1 [source] => Membership Webform [check_permissions] => [version] => 3 ) "
It looks like because there is a session for that browser, the form can't create a new contact because it is linked to the first contact id.
The only way I can see of enabling this browser to complete the form is to empty the sessions table.
I also get the error if I am logged in and select "You are viewing this form as Mr Jon Pollard. Please click here if that's not you." - and then try to create a new user.
I hope this isn't too vague.
Active
5.8
Webform/Drupal
"a membership form which logs in the user" - can you confirm how the 'log in the user' is being triggered.
It is this extension - CMS User
https://github.com/Skvare/com.skvare.cmsuser
with that extension disabled I am still getting the error.