Rabba answered: Since you require ḥalitza and you do not exempt her completely, all will know that this is merely a stringency and that the rabbis did not decide with certitude that the first betrothal was fully valid. Consequently, they would not come to disregard the other betrothal. Abaye raised a challenge: If so, let the Mishnah teach the case where it is uncertain whether the item is closer to him or closer to her with regard to divorce, and stipulate that she requires ḥalitza. And they would know that this is merely a stringency and not make a mistake.
אם אתה אומר חולצת, מתייבמת. הכא נמי, אם אתה אומר חולצת, מתייבמת! ותתייבם, ואין בכך כלום — אחזקה קא קיימא.
He answered him: A mistake could in fact be made here, as, if you say that she must perform ḥalitza then she may also enter into levirate marriage. People might mistakenly think that if she is suitable for ḥalitza then she is also suitable for levirate marriage, and as a result the woman might enter into levirate marriage, despite the fact that it is forbidden for her to do so. Abaye objected: Here too, in the case of uncertain betrothal, the concern exists that if you say that she performs ḥalitza then she might also enter into levirate marriage. Rabba answered: So let her enter into levirate marriage, and there is no problem with that. In this instance she remains with her presumptive status as permitted because she was originally assumed to be permitted and was rendered forbidden only due to our concern. However, there would be no actual transgression involved even if she were to enter into levirate marriage.