The suggestion that 'dukes' and 'counts' are some sort of neat hierarchy of rank is a post-medieval invention, a desire to map a modern fictional 'feudal pyramid' onto the medieval period. In fact, we don't know why all ducal and comital territories were known as such: some disappear into the fog of the Merovingian centuries. There are plenty of theories, but the most discreditable ones would start with a fine sense of modern order imposed on the past, a past where terms like dux and comtes appear in so many variants in Latin and vulgar texts from 7th to 11th centuries that it's bewildering.

Lands could become a duchy because the ruler was a duke, or vice versa; the same for counties, and even for marches. There were no rules to it, fixed and immutable.

By 12th century, count and duke were heritable, aristocratic titles attached to land (this is for the nascent high middle ages Francia). Most nobility had multiple titles, not just single count and duke, or sometimes both. These accrued through territorial acquisitions, like badges, reconfiguring with marriage, conquest, exchange. They weren't ranks and only meant as much as communities of nobility gave them credence. So, the Plantagenets could be kings and dukes at the same time (England and Aquitaine/Poitou, respectively), but that dukedom didn't somehow 'rank' over the count of Toulouse.