Gemara

(Mishnah): If an Ervah was Safek Mekudeshes (to a brother) or Safek divorced, the Tzaros do Chalitzah, not Yibum;

Safek Kidushin is when he threw Kidushin money to her, and we are unsure if it landed closer to him or to her;

Safek divorce is a Get in the husband's handwriting without witnesses, or it has witnesses but no date, or it has a date but only one witness.

(Gemara) Question: Why doesn't the Mishnah list among Safek Gerushin when he threw a Get to her and we are unsure to whom it is closer?

Answer #1 (Rabah): The Tzarah's Chazakah was to be exempt from Yibum or Chalitzah (when her husband will die). When in Safek, we follow her Chazakah.

Objection (Abaye): If so, we should say the same about Safek Kidushin, (e.g. Leah is Safek Mekudeshes to Reuven)!

The Chazakah of her Tzarah (Reuven's other wife) was to be permitted to do Yibum. We should not forbid her due to Safek!

Answer: There, we are stringent.

Objection: This stringency will lead to a leniency!

If later Reuven was Mekadesh Leah's sister, or another man was Mekadesh Leah (and Reuven died), since Leah's Tzarah was forbidden to do Yibum, people will think that Leah was Vadai Mekudeshes, so the second Kidushin was invalid (and Leah's sister may marry Reuven's relatives/Leah is permitted)!

Answer: Since we require the Tzarah to do Chalitzah, people realize that this is only a stringency, so they will not err.

Question: We should teach the case of Gerushin (when we don't know to whom the Get was closer) and require Chalitzah, so people will know that this is only a stringency!

Answer: If we would require Chalitzah, people would come to do Yibum!

Question: The same concern applies to Safek Kidushin!

Answer: There, it is no problem if they do Yibum. The Chazakah permits Yibum!

Question (Mishnah): If a man threw a Get to his wife in Reshus ha'Rabim... if it landed in the middle, she is divorced and not divorced.

This means that if she is Ervah, her Tzarah must do Chalitzah. We are not concerned lest requiring Chalitzah lead people to do Yibum!

Answer: Rabah and Rav Yosef established the Mishnah to be when one pair of witnesses says that the Get was closer to her, and another pair says that it was closer to him. This is a Safek mid'Oraisa;

If our Mishnah would teach such a case, it would need to discuss when there is only one pair of witnesses, which is a Safek mid'Rabanan.

Objection #1: Perhaps our Mishnah discusses Safek Kidushin when there are two pairs of witnesses!

Answer: If there were two pairs, there would be no reason not to do Yibum!

Rejection: Two witnesses who say that the money was closer to her is a great reason not to do Yibum!

Objection #2: Also when there are two pairs of witnesses, the Safek is mid'Rabanan! Mid'Oraisa, the pairs of witnesses cancel each other, and we follow her Chazakah (she is not Mekudeshes, so the Tzarah may do Yibum).

There was a man who alternated between sanity and insanity. He sold property. Two pairs of witnesses argued about whether he was sane or insane at the time. Rav Ashi ruled that the witnesses cancel each other, and we follow the Chazakah that the property belongs to the man.

Answer #2 (to question 4 - Abaye): Each clause teaches about the other. The case taught regarding Kidushin also applies to divorce. The cases taught about divorce also apply to Kidushin.

Objection (Rava): If so, what does 'This is (the case)' come to exclude?

Answer #3 (Rava): The case of Kidushin also applies to divorce. There is a case of divorce that does not apply to Kidushin.

'This is' regarding divorce doesn't exclude anything. It was taught for parallel structure with 'this is' regarding Kidushin.

Kidushin 5b: If he gave Kidushin money and she said 'I am Mekudeshes to you through this money', it is a Safek. Mid'Rabanan, we are stringent and consider her Safek Mekudeshes.

Rishonim

Rashba (31a DH u'Masnisin): Rashi says that in the case is one pair of witnesses they argue about to whom the Get was closer. This is wrong. We are not concerned for one witness who is contradicted by another witness! Rather, both witnesses are unsure to whom it was closer.

Rif and Rosh (Yevamos 8b and 3:7): If an Ervah was Safek Mekudeshes or Safek divorced, the Tzaros do Chalitzah, not Yibum. Safek Kidushin is when he threw Kidushin to her, and we are unsure if it landed closer to him or to her. Safek divorce is a Get in Kesav Yado without witnesses, or with witnesses but no date, or with a date but only one witness. Similarly, if she was standing in a Reshus ha'Rabim, and he threw a Get to her and we are unsure to whom it was closer, the Tzaros do Chalitzah, not Yibum. This is like Rava taught, that the case of Kidushin also applies to divorce.

Nimukei Yosef (DH Masnisin): We could also say that one pair of witnesses says that the Get was closer to her, and another pair says that it was closer to him. Mid'Oraisa, the witnesses cancel each other, and we follow the Chazakah that the Tzarah is exempt (regarding Safek divorce) or permitted to do Yibum (regarding Safek Kidushin). Chachamim were stringent to require Chalitzah, for it is a Safek Isur. Rava agrees with Abaye that when there are two pairs of witnesses it is a Safek mid'Rabanan.

Rif and Rosh (Kidushin 2a and 1:1): If he gave the money and she said the words, we are concerned mid'Rabanan and she needs a Get.

Ran (DH Tanu): The Gemara says 'it is a Safek, and mid'Rabanan we are concerned'. If it is a Safek, we should be concerned mid'Oraisa! The Rif omitted 'it is a Safek'. It seems that he holds that really we know that it is not Kidushin; mid'Rabanan we consider it a Safek. The Gemara connotes otherwise. The Rambam says that it is Safek Kidushin; he does not say 'mid'Rabanan'. Perhaps every Safek Kidushin does not require a Get mid'Oraisa, for we follow the Chazakah (that she is single), just like every monetary Safek. All Acharonim agree that even if the other party seized the money in doubt, we return it to the Muchzak. The same applies to Safek Kidushin, but mid'Rabanan we are stringent due to the stringency of Safek Ervah. Therefore we are concerned for every Safek Kidushin. However, in another case the Rif says that it is a Safek Kidushin mid'Oraisa and we are stringent. This requires investigation.

Rambam (Hilchos Yibum 6:22): If an Ervah to the Yavam was Safek Mekudeshes or Safek divorced, the Tzarah does Chalitzah, not Yibum.

Magid Mishneh: The Mishnah requires Chalitzah also after Vadai Gerushin with a Get that is Pasul mid'Rabanan, e.g. it does not have a date or two signed witnesses.

Poskim

Shulchan Aruch (EH 173:6): If a Yevamah was Ervah to the Yavam and she was Safek Mekudeshes or Safek divorced, the Tzarah does Chalitzah, not Yibum.

Beis Yosef (DH v'Chosav, citing R. Yerucham): This is not only when two pairs of witnesses argue about to whom the Get was closer, a Safek mid'Oraisa. Rather, even when there are only two witnesses, which is a Safek mid'Oraisa, the Tzarah does Chalitzah.

Rebuttal (Beis Shmuel 5): The Shulchan Aruch discusses a Safek mid'Oraisa, when we do not know to whom it was closer. If there are two pairs of witnesses, the Safek is mid'Rabanan! Mid'Oraisa, the pairs of witnesses cancel each other, and we follow her Chazakah. (If she was Safek Mekudeshes, the Tzarah may do Yibum. If she was Safek divorced, the Tzarah is exempt from Yibum and Chalitzah!)

Rebuttal (Beis Meir): The Beis Shmuel connotes that there is a case which is a Safek mid'Oraisa. Also there mid'Oraisa we should follow the Chazakah! Poskim argue about when there are two sets of contradictory witnesses (some say that they also cancel the Chazakah). When the witnesses themselves are unsure, the Beis Shmuel himself (30:9) cited Hagahos Ashri to say that this is like Kidushin without witnesses, so the Tzarah may do Yibum! However, the Rashba disagrees.

Support (Nesivos ha'Mishpat Dinei Tefisah b'Edim Bi'urim 7): We heed testimony only if the witnesses saw everything that they could have. If they did not verify to whom the money was closer, this is Chetzi Davar. It is totally invalid, even if other witnesses (who did not see the money thrown) testify that the money was closer to her.

Pischei Teshuvah (30:2): Hagahos Ashri does not say this, rather, the Mordechai does. Many disagree.