Makkot 14A

כדאמרינן מעיקרא, שאם עשו תשובה ⁠– בית דין של מעלה מוחלין להן. מאי אמרת? הא לא עבוד תשובה? לא פסיקא מילתא לכרת.

as we stated initially, that if they repented the heavenly court absolves them of the punishment of karet, and therefore this is not a case of two punishments for one transgression, and there is no exemption from lashes. What do you say in response, that they did not yet repent? Nevertheless, since the matter is not clear-cut with regard to karet, as perhaps they repented, they are therefore not exempt from lashes.

רבי יצחק אומר: חייבי כריתות בכלל היו, ולמה יצאת כרת באחותו ⁠– לדונו בכרת ולא במלקות.

§ It is taught in the baraita that R' Yitzḥak says: Those liable to receive karet in cases of incest were included in the principle: “For anyone who performs any of these abominations, the souls who do so should be excised from among their people” (Leviticus 18:29). And why was the punishment of karet, when administered to one who has sex with his sister, excluded from this verse and mentioned independently (Leviticus 20:17)? It is to sentence him to be punished with karet and not to be punished with lashes.

ורבנן, כרת באחותו למה לי? לחלק, וכדרבי יוחנן, דאמר רבי יוחנן: שאם עשאן כולם בהעלם אחד ⁠– חייב על כל אחת ואחת.

The Talmud asks: And according to the Rabbis, i.e., R' Yishmael and R' Akiva, who maintain that those liable to receive karet are flogged, why do I need karet to be written in the case of one who has sex with his sister; what does it teach? The Talmud answers: It teaches to divide the various prohibitions of sex with forbidden relatives, and is in accordance with the statement of R' Yoḥanan, as R' Yoḥanan says that if one performed all the transgressions described in the passage in the Torah enumerating forbidden relatives during one lapse of awareness, he is liable to bring a separate sin-offering for each and every one, as one is liable to receive karet for each and every one.