UNDOING A MA'AMAR
(The Mishnah said that if Reuven died, Shimon gave a Ma'amar, then he died, both Yevamos do Chalitzah, not Yibum.)
Version #1 (Rava): If a Yavam gave a Ma'amar to Leah and then a Get to undo the Ma'amar, her Tzarah may do Yibum. Leah is forbidden, lest she be confused with the recipient of a Get to undo Zikah.
Version #2 (Rava): If he gave a Get to undo his Ma'amar to Leah, even she may do Yibum. He undid his Ma'amar.
A YEVAMAH WHO WAS ONCE FORBIDDEN IS ALWAYS FORBIDDEN
(Mishnah): Reuven and Shimon (brothers) were married, respectively, to Rachel and Leah (sisters). Reuven died, and then Shimon's wife died. Reuven's widow is forever forbidden, since she was once forbidden.
(Gemara) Question: This is obvious!
In the Mishnah of three brothers (30a), when she was able to do Yibum with a different Yavam, she was forever forbidden to the Yavam she was initially forbidden to.
Here, she cannot (initially) do Yibum at all, all the more so, she is forever forbidden!
Answer: The Tana first taught our Mishnah, thinking that in the other case, she would be permitted. Later, he realized that even there, she is forbidden.
Since it is a bigger Chidush, he put it before our Mishnah. Our Mishnah was retained (even though it does not add a Chidush).
CAN TWO ISURIM APPLY SIMULTANEOUSLY?
(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: (He 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: Does R. Yosi really hold that Isur Chal Al Isur (a second Isur takes effect on something already forbidden)?
(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): What is considered the first that applies? If Chamoso (his mother-in-law) got married (and he had Bi'ah with her), he is liable 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).
However, 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).
R. Chiya and Bar Kapara argued as R. Yosi and R. Shimon do.
(R. Chiya): If a Zar (non-Kohen) served in the Mikdash on Shabbos, he is liable for two Aveiros;
(Bar Kapara): He is liable for one.
R. Chiya: I swear that Rebbi said that he is liable twice!
Bar Kapara: I swear that Rebbi said that he is liable once!
R. Chiya: Shabbos was forbidden to all. It was permitted (for Avodah) only to Kohanim. He is liable for transgressing Shabbos, and for Avodas Zar.
Bar Kapara: Shabbos was forbidden to all. It was permitted (Stam, i.e. to all). He is liable only for Avodas Zar.
(R. Chiya): If a Kohen Ba'al Mum served in the Mikdash b'Tum'ah, he is liable for two Aveiros; (p) (Bar Kapara): He is liable for one.
Each of them swore that Rebbi said like him.
R. Chiya: Avodah b'Tum'ah was forbidden to all. It was permitted only to unblemished Kohanim (for Korbanos Tzibur). He is liable for Avodas Ba'al Mum, and for serving b'Tum'ah.
Bar Kapara: Avodah b'Tum'ah was forbidden to all. It was permitted in the Mikdash (Stam). He is liable only for Avodas Ba'al Mum.
(R. Chiya): If a Zar ate Melikah (Chatas ha'Of; it is killed through pinching its neck), he is liable twice;
(Bar Kapara): He is liable once.
Each of them swore that Rebbi said like him.
R. Chiya: Eating Neveilah (something that died without Shechitah) was forbidden to all. It was permitted in the Mikdash only to Kohanim. He is liable for a Zar eating Kodshei Kodoshim, and for eating Neveilah.
Bar Kapara: Eating Neveilah was forbidden to all. It was permitted in the Mikdash. He is liable only for a Zar eating Kodshei Kodoshim.