In fact, we are going back a bit now, but I'm SURE the Queen only made William a Duke so that Catherine would not have to be referred to as
HRH Princess William
A la Princess Michael of Kent
The same is true of Duchess Of Sussex vs Princess Henry
More dignity. They never liked Princess Michael
None of this applied to Diana as Charles was already Prince Of Wales
She was NEVER Princess Diana though that is how she is remembered
She was HRH The Princess Of Wales
and post divorce
Diana, Princess Of Wales.