Why does the Torah see fit to add another Pasuk concerning a sister?
Rashi: To preclude the daughter of his father from a Shifchah Cana'anis and from a Nochris from the law of "Moledes Chutz", 1 since she is not fit to have Kidushin [with any man].
Ramban: Here it says that she is his father's daughter through marriage. We infer that "Moledes Chutz" above is not through marriage. 2
Ramban (citing Yevamos, 22b): To teach us that someone who transgresses, and has relations with his sister who is also the daughter of his father's wife who is not his mother, transgresses two Lavim. 3 ).
Moshav Zekenim: Before we learned about his father's daughter not through marriage. We cannot punish, from a Kal v'Chomer, for his daughter through marriage, therefore this verse teaches it explicitly.
Makos 5b: We already learned (verse 9) about his father's daughter not through marriage. Since the Torah needed to teach about his father's daughter through marriage, we infer that we cannot warn 4 via a Kal v'Chomer.
Despite the fact that one is not sure that it is his father's daughter, and despite the fact that a Nochri is permitted to marry his paternal sister (Ramban).
If he was Mezid he gets two sets of lashes; if he was Shogeg, he brings two Chata'os. (PF)