Rashi says that mei raglayim in considered tamei even if there are two reasons for safek 1) maybe it’s from an animal 2) Maybe from a person who is tahor. Generaly we know that a double safek (sfek sfeka) is permitted – why is this case different?
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See Tosfos D”H V’Al Sefek Mei Raglei . . who argues with Rash”i and states that since it is “kneged” mei raglei behama its easily “nikkur” that its from a man in which case it is only one sefek.