I think 2 Dukedoms would look like all the RF ever thought about was how many titles they had, who had the most titles.... or who was HRH, or who bowed to X or Y, that their mindset was very trivial and out of place with the 21st century. Which was IMO why Edward had his wedding at Windsor, rather than London, and it was a less grandiose wedding ("They spend a load of money on weddngs and then they get divorced!" )
So because the queen wanted him to have Edinburgh, and because she was tryng to make the RF look more in tune with the current age....she gave him an earldom and the future children were not going to have the HRH. I doubt if "Duke of Camb" was ever being proposed, if she wanted him to have Edinburgh... Possibly Ed did like the sound of Earl of Wessex.. so when told he would only have an earldom for the time being, he chose that.