When creating a new user in Salesforce, an error occurs: Duplicate Username, Another user has already selected this username.
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 to:
a) Use the Forgot Password option to log in, modify the username themselves.
b) Choose a different username.
Salesforce support is no longer able to ask administrators of a given Org to release or change a username for the sake of another Org.
Salesforce support provides Org IDs where the user already exists only if the options mentioned are not possible and after receiving authorization from the system admins of the Org that they are sharing information from.
Contact Veeva Support to open a case if the options indicated above are not possible to request for the details of the Org where the user already exists.
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