I don’t think Wallace Simpson (the divorcee who married Edward VIII) ever got a royal title. While he was allowed to serve some minor positions as Duke, I don’t beleive she ever was.
Pretty sure the former Edward VIII and Wallis were referred to as the Duke and Duchess of Windsor.
After her marriage to Edward, the Duke of Windsor, she was formally known as the Duchess of Windsor. But she was not allowed to share her husband’s style of “Royal Highness”.