פירוש על דברים 23:1

לא יקח — This does not mean “he shall not take” but “he cannot take” his father’s wife: there can be no question of a legal marriage for him in regard to her, because the marriage ceremony (קדושין) has no legal hold on her (cannot make her his wife: it is no marriage) (Kiddushin 67b; cf. Rashi on Kiddushin 67b s. v. לא יקח)

לא יקח איש את אשת אביו, this law has been repeated here to inform us that anyone born from such a union falls under the category of mamzer, “bastard.”

לא יקח איש את אשת אביו, A man must not marry the wife of his father.” The reference is to a woman whom his father had once raped. This explains why this verse is appended to the subject of rape just discussed.