I had a test class in org which has user name "test@myOrg.com'. Recently we had an issue saying that the user exist already in same or different org.
System.DmlException: Insert failed. First exception on row 0; first error: DUPLICATE_USERNAME, Duplicate Username.
The username already exists in this or another Salesforce organization. Usernames must be unique across all Salesforce organizations. To resolve, use a different username (it doesn't need to match the user's email address). : [Username]
How can I keep the user name unique?