Error: Duplicate Username, Another user has already selected this username - when creating a new user in Salesforce.
Deactivating a user is not enough to free up that username. The username must be changed in the org that it currently exists in. To determine which org contains the duplicate username, it is a best practice to check any Sandbox ORGs or related ORGs in your company for that name. Open contact Veeva Support to open a case if the user cannot be found and deleted.
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