The first Mishna in Massekhet Yevamot (2a) introduced us to the idea that when a woman is married to a man who dies without children, she cannot become the yevama of the deceased’s brother if she is closely related to him (e.g. if she had originally married her uncle, once he dies she cannot become a yevama to his brother, i.e. her own father). Furthermore, in the event that her first husband was married to another woman at the same time (a tzarah in the language of the Mishna), her rival wife will not become a yevama either.
The Mishna on our daf teaches that only Beit Hillel believes that the tzarah does not become a yevama. According to Beit Shammai, however, the tzarah is treated independently and will be subject to the rules of yibum and/or halitza as if she had been the only wife of the deceased.
While Rabbi Shimon ben Pazi seeks the source of Beit Shammai’s position in the passages that teach the rules and regulations of the mitzva of yibum and halitza, Rava suggests that it stems from the application of a well-known rule in the Talmud – ein issur hal al issur – that when there is an existing prohibition, a second prohibition cannot be added to the first. In our case, when the brother who was destined to die without children married a woman who was forbidden to the future yavam (if he married his brother’s daughter, for example), the prohibition of aishet ahiv – of marrying one’s sister-in-law – never takes effect, since this woman is already forbidden to her husband’s brother. Beit Shammai argues that in such a situation this woman is never considered a potential yevama, and so, her status does not affect her tzarah at all.
One question that is raised about this is why Beit Hillel would reject an explanation that is based on a widely accepted Talmudic rule.
The Ritva argues that this is an exception to the rule according to Beit Hillel, who derives the halakha from a pasuk (verse). Others suggest that there is a basic difference between the opinions of Beit Shammai and Beit Hillel. While Beit Shammai believes that ein issur hal al issur means that the prohibition does not exist in this case at all, Beit Hillel believes that it may not result in punishment, but it still affects the case.