Overview:
When creating a new user in Salesforce, an error occurs:
Duplicate Username, Another user has already selected this username.
Root Cause:
Solution:
Deactivating a user is not enough to free up a username. The username must be changed in the Org in which it currently exists. To determine which Org contains the duplicate username, it is a Best Practice to check any Sandbox Org or related Orgs in the company for that username.
If the Org cannot be found, Salesforce suggests:
a) Use the Forgot Password option to log in, modify the username themselves.
or
b) Choose a different username.
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.
Contact Veeva Support to open a case if the options indicated above are not viable.
Related Documentation: