Gemara

(Mishnah): Whenever Reuven may not marry Leah because he permitted her to marry (he brought her Get or testified that her husband died), if Leah married someone else and was widowed or divorced, Reuven may then marry her.

Suggestion: This is even when a widowed woman remarried and was widowed again, and a divorced woman was divorced again. Our Mishnah is unlike Rebbi, who says that a Chazakah is established after two occurrences (so a woman who was widowed twice may not remarry).

Rejection: It can be like Rebbi. The Mishnah permits a widowed woman who was later divorced, and a divorced woman who was later widowed.

64b (Beraisa - Rebbi): If a woman circumcised two of her sons and they died, she should not circumcise her third son;

R. Shimon ben Gamliel says, she circumcises the third, but not a fourth.

Question: Whom does the Halachah follow?

Answer (Rav Yitzchak bar Yosef): There was a case of a woman whose two sons died in circumcision. R. Yochanan told her to circumcise her third son.

Abaye (to Rav Yitzchak): You permit an Isur and a danger!

Abaye relied on this and married Chomah, who had been widowed twice. Abaye died in her lifetime.

Rava: Does anyone jeopardize his life like this?!

Objection #1: Abaye himself said that we may rely on Avin, who reviews his learning, but not on Yitzchak, who does not!

Objection #2: The Tana'im argue about circumcision. Do they argue about marriage? (Perhaps all forbid a two-time widow!)

Answer (to Objection #2 - Beraisa - Rebbi): If a woman was widowed twice; she may not marry again;

R. Shimon ben Gamliel says, she may marry a third man, but not a fourth.

Kesuvos 43b (Mishnah): If a girl was divorced from Eirusin and widowed from Eirusin (before Bagrus), her father gets the Kesuvah.

(Gemara) Inference: The Tana discusses one who was divorced and widowed, but not one who was widowed twice, (for this is severe,) for then she could not remarry!

The Mishnah is like Rebbi, who says that after she is widowed twice, the Chazakah is that her husbands will die.

Rishonim

Rif (Kesuvos 15a): Our Mishnah is like Rebbi, who says that two occurrences establish a Chazakah. The Halachah follows him, like it says in Yevamos that the Halachah follows Rebbi regarding Nisu'in and lashes.

Ran (DH Gemara): The Mishnah did not discuss when she was divorced twice, even though she may remarry, for then we could not infer that one who was widowed twice may not remarry.

Nimukei Yosef (DH b'Nisu'in, citing the Ritva): We do not allow one to marry a Katlanis and harm himself. This is forbidden, akin to suicide. Beis Din excommunicates him until he divorces her. Our Rebbi did so in practice.

Nimukei Yosef (DH she'Meshalem): It is a Safek whether Chazakah is after two or three times. We are stringent about a Katlanis, for it is dangerous.

Rambam (Hilchos Isurei Bi'ah 21:31): If a woman was widowed from two men she may not marry again. If she married a third man she may remain with him.

Magid Mishneh: The Gemara fixed the Halachah like Rebbi.

Note: Rava (Yevamos 64a) says that Rav Yosef rules like Rebbi regarding Nisu'in 'like we said', because the Stam Mishnah is like Rebbi. The Mishnah discussed a barren woman. Perhaps 'Nisu'in' also refers to Katlanis. We inferred that a Mishnah (Kesuvos 43b) holds like Rebbi regarding Katlanis.

Kesef Mishneh: In a Teshuvah, the Rambam says that Katlanis is not a real danger or Isur. It is a mere Nichush, fear and panic prone to strike physically weak people. It is like eating vegetables straight from the bundle (some say that this makes one vulnerable to witchcraft - Shabbos 82a). Even though some say that she should not marry, if a man is Mekadesh her we will not make him divorce her. This was done by the Batei Din of the Rif and R. Yosef ha'Levi, and we do so in Mitzrayim. Regarding Yibum, no one ever protested.

Maharam Alashker (79, citing Teshuvas ha'Rambam): In all cities of Sefard we permit a young Katlanis lest she turn to Zenus. Chachamim ignored the Isur to free slaves when there was concern for Pritzus with Shifchos.

Rosh (6:13): Presumably, Chazakah is established after two times regarding Milah and Katlanis, because we are lenient about Safek Nefashos (i.e. we rely on the opinion that minimizes the danger).

Poskim

Shulchan Aruch (EH 9:1): If a woman was married (Rema - or Mekudeshes) to two men and they died, she may not marry a third, for her Chazakah is that her husbands die.

Note: Even though the Mechaber says 'Niseis', which connotes Nisu'in, surely he does not exclude Kidushin. (See Beis Yosef DH v'Ha.) The Rema explains that the Mechaber includes Kiushin.

Beis Yosef (DH uvi'Terumas, citing Terumas ha'Deshen 211): Even Chachamim and great people are not careful about this. This is difficult, for the Rosh says that that we force her third husband to divorce her. Even the Rambam forbids her to marry l'Chatchilah. Perhaps they rely on the Or Zaru'a, who permits her to marry a third time. It is only a Safek danger. There are other things that Chachamim considered dangerous which we are not careful about. Especially nowadays that Yisrael is few in number, we must marry when we can. Since many people do so, we rely on "Shomer Pesa'im Hash-m."

Rebuttal (Terumas ha'Deshen, ibid.): It is not clear why a Chacham, or even a commoner who knows the problem, may rely on "Shomer Pesa'im Hash-m." Perhaps we are lenient due to young Katlaniyos, lest they adopt evil lifestyles if we forbid them to remarry.

Rema: Some say that this is only if they died normally, but if one was killed, died through Dever, fell from the roof or similar matters, we are not concerned. Therefore, many are lenient about these and we do not protest.

Beis Yosef (DH veha'Rivash): The Rivash says that many people were lenient when one husband died through Dever, and the Gedolei ha'Dor did not protest. We do not attribute Makas Medinah to Mazel (Bava Metzi'a 105b); however, this is only if the majority of the city died. Perhaps it suffices for three deaths for each 500 men (fitting to go to war) in the city on three consecutive days (we fast over such Dever). If they died from a different sickness, one should be stringent, for the Gemara (Bava Metzi'a 106a) did not resolve a similar question.

Note: The Gra (2) bring a support for the Pesak that one need not divorce a Katlanis from Yehudah, who kept Tamar (Sotah 10b). This suggests that Katlanis applies even when her husbands were killed (by Hash-m) for their Aveiros! However, Yehudah did not have witnesses saying why they died, so perhaps we may infer that in any case one may keep her.

Darchei Moshe (2): Some say that if one of the husbands was old, he does not count towards making her a Katlanis. However, if all her husbands died from the same sickness even the Rambam would force a future husband to divorce her.

Pachad Yitzchak (Katlanis): The Rema himself does not distinguish whether or not one was old, therefore he does not discuss this in the Shulchan Aruch.

Shulchan Aruch (2): If a man married twice and his wives died, he may remarry.

Source (Beis Yosef DH Ish): The Rosh says that if this were forbidden, Chachamim would have mentioned it.