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Cognomen Kastor (Kastor , Кастор) , in all the world, is quite a rare last name. The surname Kastor is habitual in Estonia, where it is quite a rare surname, Algeria, where it is a rare surname, and Malaysia. Measured by absolute frequency, it is the most numerous in the United States, Algeria, and Germany. Likewise, Kastor is the forename as well as the last name.