Here's the list of duke and significant vassals, with some notes
The duchy of Brittany was divised in some earldoms (comtés), which was for many of them former kingdoms of early breton settlement, and for others former "marches" of carolingian era (Nantes and Rennes)
there was the comtés of :
Nantes
Rennes
Vannes
Cornouailles (Quimper)
Léon
Penthièvre
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Ruler Conan II de Rennes
Title: Duc de Bretagne
Vassal of: nobody (sovereign)
Current location: Rennes
Was born at: 1.1.1037
Age to game start: 29
Family status: Single
Father: Alain III Ruibriz de Rennes (996-1040)
Mother: Berthe de Blois (1013, still alive in 1066)
#see footnote 1
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Ruler Geoffroi Grenoat de Rennes
Title: Comte de Rennes
Vassal of: Duc de Bretagne
Current location: Rennes
Was born at: 1.1.xxxx
Age to game start: xx
Family status: Single
Father: Alain III Ruibriz de Rennes
Mother: Unknown
#2
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Ruler Hoël V de Cornouaille
Title: Comte de Nantes
Vassal of: Duc de Bretagne
Current location: Nantes
Was born at: 1.1.1010
Age to game start: 56
Family status: Married
Spouse: Havoise I de Rennes
Father: Alain III Canhiart de Cornouaille
Mother: Judith de Nantes
#3
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Ruler Hoël V de Cornouaille
Title: Comte de Cornouaille
Vassal of: Duc de Bretagne
Current location: Kemper
Was born at: 1.1.1010
Age to game start: 56
Family status: Married
Spouse: Havoise I de Rennes
Father: Alain III Canhiart de Cornouaille
Mother: Judith de Nantes
#4
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Ruler Alain I de Léon
Title: Vicomte de Léon
Vassal of: Duc de Bretagne
Current location: Kastell-Paol
Was born at: 1.1.xxxx
Age to game start: xx
Family status: Married
Spouse: unknown
Father: Morvan de Léon
Mother: unknown
#5
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Ruler Eudes de Penthièvre
Title: Comte de Penthièvre
Vassal of: Duc de Bretagne
Current location: Lamballe
Was born at: 1.1.999
Age to game start: 67
Family status: Married
Spouse: Onguen de Cornouaille
Father: Geoffroi I Bérenger de Rennes
Mother: Havoise de Normandie
#6
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Footnotes
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1 - Conan died December 1st 1066
2 - Bastard, brother of Conan
3 - Havoise is a sister of Conan and Geoffroi. She became Duchesse de Bretagne when Conan died, and Hoël became Duc consort.
4 - The possesions are distincts
5 - His son Guiomarch II de Léon was part of William's expedition in 1066, ans was senechal of Richmond
6 - Uncle of Conan, Geoffroi and Havoise