I will reply more later, but it seems that lifting barons to the rank of Duke was very uncommon or unknown in 12th century England. Earls were considered the highest noble rank before the King (bishops and Archbishops are up there too) and I have yet to find an instance of a Duke prior to the late Middle Ages. In effect Earls were the highest temporal rank of landed aristocracy. In addition there were many times when there were few enough of them - I know that at the time of the battle of Bannockburn (early 14tb century) the lack of earls was cited as a reason for the failure of the English army.

Earls as a rank was derived from the Anglo-Saxon Ealdorman in function and the Scandinavian Jarl in name. In practice however these two ranks had been very similar before their combination. Prior to the Norman conquest some Earls rivaled the power of the King - Harold Godwinson came from the prestigious house of Wessex, and was tied to the royal line of Alfred the great. He controlled large swathes of southern England in his capacity as Earl. In some ways he was so powerful that he would resemble later dukes, though by the time of the Dukes, responsibilities and the Royal relationship with the nobility had changed significantly. In any case the Normans continued many of the traditions of the landed aristocracy in terms of responsibilities and nomenclature, for example the shire reeves or sheriffs.

The Ducal title is primarily continental in origin and stems from the Latin 'Dux,' or leader. It has been replicated in other languages, for instance the Serene 'Doge' of Venice or when Mussolini called himself 'Duce'. The fact that William the conqueror was the 'Duke' of Normandy stemmed from the French kings agreements with his forbears, and had little to no bearing on the nobility of England until much later than the 12 century.

In sum I would say that the title of Earl represented a noble who could at times fill the place between the continental titles of 'Count' and 'Duke'. In practice they were more similar to Counts in terms of responsibilities and areas under their control.

For reference I would include Frank Barlow's The Feudal Kingdom of England, 1042 to 1216. I will add more sources later.