She is, unless Harry is stripped of the title (which would take an Act of Parliament) or disclaims it. It would not be possible for the King to grant the same title to someone else again, but it would be possible for the King to create a new legally different but similar-sounding peerage. An example for this is the Earl of Oxford and Earl Mortimer, which, despite the odd form, was one title, and it was created because a title of Earl of Oxford already existed. To complicate things further, yet another peerage, the Earl of Oxford and Asquith, was also created at a later time. So presumably, the King could create a new title of Duke of Sussex and Somethingsomething, arguing that it’s distinct from Duke of Sussex. But it would probably be seen as cheeky.