How to resolve DUPLICATE_USERNAME error returned while performing a deployment

Usernames are shared across instances, but not across environments (production/sandbox). I.e. if a user account in a sandbox instance has username = a@b.c, this username cannot be used in any other sandbox instance (CS*), but it can be used in production instances (NA*, EU*, AP*)

Uniqueness is enforced during deployments when tests are run, so an insert call will fail if the username is already registered in another org in the same environment (production or sandbox). The resolution for this is to use a guaranteed unique username for tests. We recommend generating globally unique usernames, e.g. org Id + timestamp + random value, as the below code snippet shows:


public static User createTestUser(Id roleId, Id profID, String fName, String lName) {
    String orgId = UserInfo.getOrganizationId();
    String dateString = String.valueof(Datetime.now()).replace(' ','').replace(':','').replace('-','');
    Integer randomInt = Integer.valueOf(math.rint(math.random()*1000000));
    String uniqueName = orgId + dateString + randomInt;
    User tuser = new User(  firstname = fName,
                            lastName = lName,
                            email = uniqueName + '@test' + orgId + '.org',
                            Username = uniqueName + '@test' + orgId + '.org',
                            EmailEncodingKey = 'ISO-8859-1',
                            Alias = uniqueName.substring(18, 23),
                            TimeZoneSidKey = 'America/Los_Angeles',
                            LocaleSidKey = 'en_US',
                            LanguageLocaleKey = 'en_US',
                            ProfileId = profId,
                            UserRoleId = roleId);
    return tuser;
}