Salesforce Error 'Duplicate Username' When Creating or Editing a User

Because ‌Salesforce usernames are unique for all users, the "Duplicate Username" error can occur when adding or editing users.  A username can exist in both sandbox and production, but duplicates are not allowed in each instance type. For more information, see Guidelines for Adding Users.

Keep in mind that for security reasons, Salesforce Customer Support can’t change usernames or deactivate users from an org. This must be done by the user or by an Administrator within the organization.  If you get a "Duplicate Username" error, check if the username is already in use in your production, trial, or Sandbox orgs. Deactivate or change the username for the user record, then create your account with your desired username in your production org. If you’re unable to change or deactivate a username, contact your Salesforce admin for help. If you’re unable to locate the org where the username is already in use, try a different username to create your account.

If receiving this error,  go through all of your Orgs and find where the username is in use.  Don't forget to check your inactive users because a username assigned to an inactive user is not available for use. If it is determined that the username is being used in an org not associated with your account, Salesforce Support is unable to provide information on where the username is being used. This security is in place to ensure all Salesforce customer data is secure. 

Tips and Troubleshooting Duplicate Usernames

• Duplication rules also apply to Communites/Digital Experiences.  Please see Communities User Licenses.
•  You can locate a Username across Orgs using the Environment Switcher.  If you know the email the user might have used, you can log in to the "Salesforce Environment Switcher" and potentially find all the orgs where this email is being used.  For instructions on using the Environment Switcher to verify the identity with the email address, see Log In with an Email Address Associated with Multiple Accounts.
• A Salesforce username does not have to be the user's email address. For more information, please review Salesforce username does not have to be your email address. Users have an additional field named email address, which doesn't have to be unique. That is, multiple users can share the same email address.  The email address must be current and valid as this is where you will receive password resets, etc.
• Search for the Username in the affected user's email inbox where they can look out for the verification email with the particular username. The Subject of the email is "Welcome to Salesforce: Verify your account". This can help in identifying if a user record with the same username was created earlier. 
• If you have a trial account, the username can be changed by the person that has signed up for the trial.
• Production and Sandbox are different server sets. ‌ User duplication is not allowed on the production server set or on the sandbox server set. The two are not referenced to each other, though, because of the sandbox copy. It is expected that a system admin or user would want the same username for a sandbox. This does not overlap because sandbox requires use of test.salesforce.com to login and production servers use
  the login.salesforce.com server.

Change Usernames

For information on changing usernames, if you are an administrator, see Edit Users.  For user updates, visit Update Personal Information.