Salesforce Error 'Duplicate Username' Creating a Username

 Keep in mind that Salesforce Customer Support can’t change usernames or deactivate users from an org. 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.

• Salesforce usernames are unique for all users, across all production or sandbox organizations and instances. 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.
  • This also applies to Communities/Digital Experiences. Please see Communities User Licenses.
• 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. For a user, there's an additional field named email address, which can be a duplicate. That is, multiple users can share the same email address. 
• If receiving this error,  go through all of your Orgs and find where the username is in use.
• In addition, please search the Username in the affected user's email inbox where they can lookout 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 same username was created earlier. 
• Also, check your inactive Users. A username assigned to an inactive user is NOT available for use.
• For security reasons, Salesforce personnel can't change usernames - this change must be done by the user or by an administrator within the organization. For trial accounts, it'll be the person that has signed up for the trial.
• If it's determined that the username is being used in an org not associated with your account, for security purposes, Support is unable to provide information on where the username is being used. This ensures that all Salesforce customer data is secure.
• The username and email address may be different. The email address should remain the same, as that field does need to be a valid and current email address. That's where you'll receive password resets, etc.
• 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

Note: 

To change the username the 'Manage Users' permission is required. Please contact your System Administrator for help if lacking this permission.

User update

1. Search the name of the user in global search.
2. Click the user's name.
3. On the right side, click User Detail.
4. On the User Detail page, click Edit.
5. Modify the username.
6. Click Save.

Note:

If you encounter the duplicate user name using Lightning Essentials, the Admin will need to switch to Classic in order to create the User as the Lightning interface in Essentials will not allow access to this field.

Administrator update

TIP : Locate Your Username Across Orgs Using the Environment Switcher

Quick workaround with the feature Access Multiple Salesforce User Accounts (Reference article) can be used to manage your orgs. You can choose to log in via email. (Only within the Organizations)

If you know the email the user might have used, they can log into the "Salesforce Environment Switcher" and potentially find all the orgs where this email is being used.
Which could give you a hint on where the specific username is being used and contact the right person directly.
Verify Your Email Address and Log In : 

1. Log in to one of the user accounts associated with your email address.
2. Open the Log In with Email page.
3. Enter your email address, and then click Log In.
4. The Verify Your Identity page opens, and a verification code is sent in an email to the address that you entered.
5. Copy the verification code from the email, and enter it in the fields on the Verify Your Identity page.
6. After your account is verified, the Environment Switcher opens with a tile for each of the Salesforce properties that you can access.