Require Unique Usernames for Partners at the Org Level
You are no longer required to create a username that is different from all the usernames ever created, when adding someone to a community. Select Require unique usernames for partners in this org to contain the unique username requirement to one org and one org only. After you enable this preference, it can’t be reversed.
Where: This feature is available in Lightning and Salesforce Tabs + Visualforce communities accessed through Lightning Experience and Salesforce Classic. Communities are available in Enterprise, Performance, Unlimited, and Developer editions.
How: New orgs created after Winter ‘19 contain username uniqueness at the org level. Contact Salesforce Customer Support to enable this option in orgs created before Winter ‘19. Go toto turn on this preference. This option requires partners to provide their org IDs when logging in through an API.