This is a common situation where employees with one part of the name are the same with a number of employees especially the first name.
When you create a user name that complains about the username already exists, it is usually the case where another user is holding the same user name.
It is also possible that a past employee could have the same user name as a new user that is to be created.
To overcome this error, the user name could be a combination of the first name with the last name or middle name.
Refer below article