jordarkelf said:"Battle" is just like "Graviate" a mediaeval subdivision higher than county, but lower than king/prince/etc..
Ergo, the second tier, implying the title above them is the third tier -- or rather that pesky "archduke" tier we'd need.
But those 'graviates' didn't have the power that dukes have in CK. That is f.e. the reason why the Irish kings are all 'dukes'. The fact that no 'duke' is known (and the 'duchies' existed for over 100 years, as you say) should indicate that there power was very limited. Probably just an administrative matter and not a 'fuedal' division of the land.
Brittany's loyalty/alliegance switched between France and England, a thing that can best be represented by keeping it a duchy. Since then you can switch alliegances.
Brittany can't be compared to Bohemia since Bohemia did have kings, while Brittany had not, so that is also not a good example. Or just as bad an example as you claim Flanders is.