@richardshagrin
Can there be a Duchess of Normandy?
The whole thing is unclear, because of
the Treaty of Paris of 1259, when the English sovereign ceded his claim except for the Channel Islands; i.e., the Bailiwicks of Guernsey and Jersey, and their dependencies (including Sark).
So after this treaty a tiny part of the duchy remained with the crown, while the whole mainland part became French and the French kings bestowed the title to French subjects while the British monarch still claimed the title "Duke of Normandy". Queen Victoria was Duke of Normandy as is Queen Elizabeth. Therefor no "Duchess of Normandy".
But
Thereafter, the ducal title was held by several French princes.
In 1332, King Philip VI gave the Duchy in appanage to his son John, who became king John II of France in 1350. He in turn gave the Duchy in appanage to his son Charles, who became king Charles V of France in 1364. In 1465, Louis XI, under constraint, gave the Duchy to his brother Charles de Valois, Duke of Berry. Charles was unable to hold the Duchy and in 1466 it was again subsumed into the crown lands and remained a permanent part of them. The title was conferred on a few junior members of the French royal family before the abolition of the French monarchy in 1792.
If one of these Duc de Normandy was married, his wife was 'la Duchesse' which translates to Duchess. Since 1792 the French title was not used anymore, so today no "Duchess of Normandy".
I can speculate about a Duchess where a future Queen has a same sex marriage and her spouse become Duchess while she is Duke of Normandy.
HM.
(typo edited Fench - French)