Gemara

(Beraisa #1 - R. Yosi): If Shimon had Bi'ah with Rachel (before Leah died), he is liable for the Isurim of Eshes Ach and Achos Ishto;

R. Shimon says, he is liable only for Eshes Ach.

Contradiction (Beraisa #2 - R. Shimon): He is liable only for Achos Ishto!

Answer: (R. Shimon always holds that only the first Isur takes effect.) In Beraisa #2 Shimon married (Leah) before Reuven married (Rachel, so the Isur of Eshes Ach took effect first). In Beraisa #1, Reuven married first.

Question: According to R. Shimon, when Reuven married first, since the Isur of Achos Ishto does not take effect, Yibum should be permitted!

Answer (Rav Ashi): The Isur is pending. If the Isur of Eshes Ach would go away, the Isur of Achos Ishto would take effect. Therefore, Yibum is forbidden, and the Isur of Eshes Ach remains.

Question: Elsewhere, R. Yosi holds that Ein Isur Chal Al Isur!

(Beraisa): If a man did an Aveirah punishable by two different deaths, he receives the more severe one;

R. Yosi says, he is sentenced to the first that applied.

(Beraisa): (R. Yosi holds that) if Chamoso (his mother-in-law) got married, he is liable for her for Chamoso. If she was married and then became his mother-in-law, he is liable for Eshes Ish.

Answer #1 (R. Avahu): R. Yosi agrees that that Isur Chal Al Isur if the latter Isur is Mosif (it forbids the forbidden object to more people).

Objection: This explains the case when Shimon married before Reuven did. When Rachel married Reuven, she became forbidden to all his brothers, so the Isur of Eshes Ach also takes effect on Shimon (her sister's husband).

But when Reuven married first, why is Achos Ishto an Isur Mosif (when Shimon marries Leah? Rachel does not become forbidden to more people!)

Suggestion: It is Mosif because Shimon became forbidden to all the sisters.

Rejection: This is an Isur Kolel (it forbids more objects on the person)!

Answer #2 (Rava): Really, he is liable only for one. R. Yosi means that he is considered liable for both. This teaches that he is buried among the utterly wicked (like one who did two Aveiros).

Kerisus 14b (Mishnah): If (Esav) has relations with his daughter's daughter (Plonis), he can be liable for this and for his daughter-in-law, his brother's wife, his father's brother's wife, a married woman, and a Nidah. (The case is, at different times Plonis married Esav's son, brother, and uncle.)

R. Yosi says, if Esav's father (Yitzchak) had transgressed and married Plonis, Esav will also be liable for Eshes Av.

(R. Avahu and Ravin): Elsewhere, R. Yosi holds that Ein Isur Chal Al Isur. Here he says that Eshes Av is Chal because it is an Isur Mosif.

The case is, Yitzchak has another son (Yakov) - since Eshes Av forbids Plonis to Yakov, it also takes effect on Esav.

Rishonim

Rif (Yevamos 10a): Some say that the Isur Bas Achas in R. Meir's Mishnah of Chelev is for eating Chelev b'Tum'ah; the case is, he was Tamei when he became Bar Mitzvah. This is wrong. Rather, R. Meir is Mechayev twice for Hotza'ah on Yom Kipur that is Shabbos, and these come b'Vas Achas.

Rambam (Hilchos Isurei Bi'ah 17:8): A great Kelal regarding all Isurim is that Isur Chal Al Isur only if they come together, or if the latter is Mosif other things on the first Isur, or it is Kolel other things together with the latter Isur.

Rambam (Hilchos Shegagos 4:1): If one did one act that transgresses several Aveiros, he is liable for each, but only if they came together, or if they were Isurei Mosif or Kolel.

Rambam (3): An Isur Mosif of Ervah takes effect only if there is another person in the world who becomes forbidden. If there is no such person, we do not say that since if Yakov would have sons or brothers Plonis would become forbidden to them she gets an additional Isur to Yakov in any case. (E.g. if Yakov's only brother married Yakov's wife's sister, Eshes Ach does not forbid her to anyone to whom she was permitted beforehand.)

Source (Kesef Mishneh): Kerisus 14b says that Eshes Av takes effect on Esav in any case when he has a brother (to whom Plonis was permitted).

Question: Since Plonis was married to Yitzchak's brother, Yitzchak can marry her only through Yibum. If so, Plonis was already forbidden to Yakov (his uncle's wife), just like to Esav!

Answer (Ri Korkus): The Rambam (Perush ha'Mishnayos Kerisus 3:5, brought in Tosfos Yom Tov DH R. Yosi) says that we must say that Yakov was a minor, so the Isur of Eshes Aviv takes effect together with Dodaso when Yakov becomes Bar Mitzvah. The Gemara in Kerisus said that we must say that Yitzchak has another son. This is not necessary. Perhaps these words of the Gemara are really from Ge'onim or Savora'im. Nevertheless the law is true that Isur Mosif takes effect only if there is someone else in the world to whom she becomes forbidden. The Ramban (Yevamos 33a DH b'Mai) says that R. Chiya (who says that R. Yosi holds that Isur Chal Al Isur if the latter is Kolel) says that R. Yosi said that Chamoso does not take effect on Eshes Ish in a case when it is not Kolel, i.e. her daughter has no other relatives who become forbidden through her Kidushin.

Mayim Chayim (in Likutim on Kerisus 3:5): R. Yosi holds that if Plonis is forbidden to Levi, she is forbidden additionally to him by an Isur Mosif only if she becomes forbidden to others due to the same Isur. In the Mishnah Torah the Rambam rules unlike R. Yosi. As long as she becomes forbidden to someone else for any reason, this is considered Mosif regarding Levi.

Rambam (Hilchos Sanhedrin 14:4): Anyone who is liable two different Misos receives the harsher one, whether he transgressed one Aveirah after the other, or one Aveirah for which he is liable two Misos.

Rambam (Hilchos Ma'achalos Asuros 4:17): Meat of different Neveilos joins (to a k'Zayis to be liable for eating it), but it does not join with meat of a Tamei animal.

Rebuttal (Ra'avad): This is like Rav and Rav Asi. A Beraisa supports Levi, who argues with them (Me'ilah 16a)!

Defense (Magid Mishneh): The Rambam holds that Ein Isur Chal Al Isur, unless it is Kolel, Mosif or Bas Achas. This is like R. Yehudah against R. Meir. If the Ra'avad disagrees, he should argue in many other places! Surely, Neveilah is a different Lav. If the Amora'im argue about Isur Chal Al Isur, the Beraisa is no support for Levi (it is like R. Meir)!

Lechem Mishneh: The Ra'avad holds like Tosfos (Me'ilah 15b DH Omar), who says that Levi agrees that normally Ein Isur Chal Al Isur. "Chelev Neveilah...Lo Sochluhu" teaches that the Isur Neveilah is Chal on Chelev, and likewise it is Chal on the Isur of a Behemah Teme'ah. The Rambam holds like Rashi, who says that the Beraisa is difficult for Rav Asi, but not for Rav.

Poskim

Shulchan Aruch (OC 485:1): If one swore not to eat Matzah, he may not eat Matzah (even) on Seder night.

Taz (1): An oath to passively Mevatel a Mitzvah takes effect through Kolel, i.e. if it also forbids Reshus (eating Matzah at other times).

Shulchan Aruch (YD 215:5): If one vowed not to eat Neveilah or something he forbade on himself through a Shevu'ah, it is not Chal. If he eats it, he is liable only for Neveilah or the Shevu'ah. If one swore not to drink wine and later vowed to be a Nazir and drank wine, he is liable only for the Shevu'ah. If he permitted his Shevu'ah, he is liable for transgressing Nezirus.

Source (Teshuvas Rashba 1:615): Nedarim are Chal to forbid doing a Mitzvah, e.g. I will not eat Matzah. A vow not to eat Neveilah is not Chal, because Ein Isur Chal Al Isur. The Yerushalmi says that a Neder is Chal on a Shevu'ah. This is because a Shevu'ah does not forbid the object, it only forbids himself (to eat it). A Neder is Mosif an Isur on the object.

Gra (11): The Shulchan Aruch says that Neder is not Chal on a Shevu'ah, unlike the Yerushalmi. This requires investigation. It is not difficult why Nezirus is not Chal on a Shevu'ah, for the Shulchan Aruch (206:5) considers Nezirus a Shevu'ah.

Question: Nezirus should be Chal on a Shevu'ah, for it is Kolel Isurim on grape pits and skins and shaving and becoming Tamei!

Answer (Zichron Yosef YD Sof Siman 26, cited in Pischei Teshuvah 2): Indeed, the Shulchan Aruch gives a mere example. Really, it discusses when it is not Kolel, e.g. he swore not to drink a cup of wine, and then said 'I am a Nazir from it'.

Note: Alternatively, we could say that he already swore not to consume anything from grapevines or shave or become Tamei, and then accepted full Nezirus.

Shulchan Aruch (6): Some say that if one vowed not to eat forbidden things such as Neveilah or Tereifah, it is Chal. If he eats it he is liable (also) for Lo Yechal.

Beis Yosef (DH Nodar): The Tur says that it is Chal. The Mishnah in Nedarim (16a) connotes like this. It seems that Nedarim are Chal on Mitzvos. Rashi (Reish Nazir) also says so. However, the Rashba says that all the Poskim disagree.