One part of this issue is due JBPAPP-7760. There isn't any check if user already exists when new one is creating.

Next part is new - I think is it useful and meaningful if we can create two users with the same user name but each belongs to different Realm, but in this case pass authentication dialog in Web Admin console isn't possible even for user belongs to ManagementRealm realm.

Also when I've created two identical users (both under ManagementRealm, it is possible now due to JBPAPP-7760), no one can pass the same step as above even they have the same password.