לֹא־יִקַּ֥ח אִ֖ישׁ אֶת־אֵ֣שֶׁת אָבִ֑יו וְלֹ֥א יְגַלֶּ֖ה כְּנַ֥ף אָבִֽיו׃ (ס)
A man shall not take his father’s wife, and shall not uncover his father’s skirt.
Rashi on Deuteronomy
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)
Rashbam on Deuteronomy
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.”
Rabbeinu Bahya
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.