Inviting a new user to an organization fails and returns the following error:
Error: Duplicate username detected and auto-adjusted
The added username is an existing ArcGIS public account. This is an expected behavior when adding a username from an existing ArcGIS public account to an organization. In some cases, the account can be considered a public account after the deletion of the account from an organization. There are two possible scenarios where the deleted account becomes an ArcGIS public account.
Refer to ArcGIS Online: Delete member for more information regarding deleting accounts.
The following procedure describes how to retrieve the username information and converting the public account into an organizational account.
Note: The following procedure may require the assistance of a user with administrator privileges or an IT personnel.
Note: If the username does not exist within the ArcGIS Online database, the credentials are not sent to the associated email. In this case, the email may be incorrect or a new ArcGIS Online account with a different username can be created to receive the invitation into the organization.