Some companies have predefined usernames and sometimes because of particular reasons, one username may need to be recycled or reused.
A couple of instances where this occurs is:
As a workaround, you can edit the inactive User record and add a number to the end of the username. This change allows you to reuse the username.
In Salesforce, usernames must be globally unique and cannot be deleted. To free up a username associated with an inactive user record, modify the inactive user's username slightly (e.g., append a number), which makes the original username available for reassignment.
By doing this, the old username retains all its records and activity logs under a different but similar username while the original username becomes available for use.
Edit Users
Considerations for Editing Users
Guidelines for Adding Users
Error 'Duplicate Username' when you create a username
000385815

We use three kinds of cookies on our websites: required, functional, and advertising. You can choose whether functional and advertising cookies apply. Click on the different cookie categories to find out more about each category and to change the default settings.
Privacy Statement
Required cookies are necessary for basic website functionality. Some examples include: session cookies needed to transmit the website, authentication cookies, and security cookies.
Functional cookies enhance functions, performance, and services on the website. Some examples include: cookies used to analyze site traffic, cookies used for market research, and cookies used to display advertising that is not directed to a particular individual.
Advertising cookies track activity across websites in order to understand a viewer’s interests, and direct them specific marketing. Some examples include: cookies used for remarketing, or interest-based advertising.