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Having to many titles may also be a problem.
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Or they might have a nicer heir so yeah olde execution might be doable. Once you win revoke the titles and take the land yourself or give it to more loyal vassals. Get claims on their lands and provoke them into rebellion. Merchant republic vassals also don't like you much. Some vassals just going to hate your guts due to traits or things like ambitious. You can ally the loyal/friend dukes via marriages etc.
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This means that you don't want your dukes to have to much land, you want lots of counts ruled by a duke.
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