Example:
One user was created in SFDC with username as abs@test.com with id as ID001.
Whenever you sync the user data with it will create a new user and SFDC User ID will be updated as ID001
Now, the customer changed the username to abc.inactive@test.com and inactivated the user
and then created a new user with the same username as abs@test.com with id as ID002.
Now if you run the sync SFDC User ID in MDA user object not updating as ID002. Because already we have a user in MDA with the same username.
So the new user unable to access the Gainsight data as he was not loaded with new SFDC ID
When syncing users, it would make more sense to sync based on ID rather than Username as usernames can be changed on records. It was explained to me that the decision was made to sync on Username so that it could be agnostic of which CRM/ID provider generated the users but I would think that every identity provider includes IDs as well as a Username for each record.
1. Update username from abs@test.com to abc.inactive@test.com for sfdc id ID001 in user management. Deactivate the user (if required).
2. Run the user sync. This will create new user abs@test.com/ID002