"The Duke of Middlesex is here in his yacht"
'View of the French Riviera' (between 1912 and 1918) - Credit: Fausto Zonaro
As might be expected, Daphne du Maurier has been careful to choose a title which has never existed in the British peerage
Having said that, the French Riviera was a popular destination for the British upper classes from the beginning of the 19th century until the early decades of the 20th, so the presence of a British Duke in Monte Carlo would not have seemed inappropriate in the period in which Rebecca is set.
The title the Earl of Middlesex has been created on two separate occasions in English history, but became extinct in 1720 when Lionel Cranfield Sackville, 2nd Earl of Middlesex became Duke of Dorset.