It’s possible for the User Management component to create duplicate users. The problem seems to be that when the User Management component is checking for valid user names it’s only validating against the user names it has loaded. So the problem arises when you filter out a user names. When you filter out user names, the User Management doesn’t check those filtered out names because they are not loaded into the User Management component. This also makes me wonder it’s possible to create duplicate users without filter out names, such as in a case where two clients have the User Management component opened and they both try to create the same user name at the same time.
This is possible using a database User Source, I did not check this against other User Source types.
I created a work around for the time being, but it might be good to update the User Management component so that it checks the User Source before saving a new user.