Denticon allows a username to be used only one time per any user. The username must be unique and unused by any other Denticon user in the world, no matter if the username is for an active, inactive, or deleted user.
The office will need to slightly adjust the desired username so that it is unique, in the same way a person often must try a few times to find an online username or email address that is not already in use in the world.
For this reason, if the user is released from employment but could potentially return to the office, users should be inactivated, rather than deleted. If a user is only inactivated, the same desired username would be available for reactivation.