Yevamot 51A

מאי טעמא דרבן גמליאל ⁠— דמספקא ליה גט אי דחי אי לא דחי. מאמר ⁠— אי קני אי לא קני. גט ⁠— אי דחי אי לא דחי. אי קמא דחי ⁠— בתרא מאי קעביד. אי קמא לא דחי ⁠— בתרא נמי לא דחי.

What is the reason for the ruling of Rabban Gamliel that a bill of divorce is not effective after a bill of divorce? It is because he is uncertain with regard to a bill of divorce whether it effectively precludes levirate marriage or whether it does not preclude levirate marriage. Similarly, he is uncertain with regard to levirate betrothal, whether it effectively acquires the yevama or does not acquire her at all. The Talmud clarifies: With regard to a bill of divorce, he is uncertain as to whether it precludes levirate marriage or does not preclude it. If the first bill of divorce precludes levirate marriage, what did he do by giving the latter bill of divorce, as it has no substance? Alternatively, if the first bill of divorce does not preclude levirate marriage, neither does the latter preclude levirate marriage.

מאמר אי קני אי לא קני, אי קמא קני ⁠— בתרא מאי קעביד, ואי קמא לא קני ⁠— בתרא נמי לא קני.

Likewise, with regard to levirate betrothal, he is uncertain as to whether it acquires the yevama or does not acquire her. If the first levirate betrothal effectively acquires the yevama, what does the last one accomplish? And if the first one does not acquire her, the last one also does not acquire her. For this reason Rabban Gamliel maintains that a bill of divorce is not effective after a bill of divorce was given, and similarly levirate betrothal is not effective after levirate betrothal was performed.

איתיביה אביי: ומודה רבן גמליאל שיש גט אחר מאמר, ומאמר אחר הגט. וגט אחר ביאה ומאמר, ומאמר אחר ביאה וגט.

Abaye raised an objection to his opinion from a baraita: And Rabban Gamliel concedes that a bill of divorce is effective after levirate betrothal and levirate betrothal is effective after a bill of divorce. If a yavam gave a bill of divorce to one yevama and then performed levirate betrothal with the other, or the reverse, both actions would be effective. And he also concedes that a bill of divorce is effective after sex and levirate betrothal such that if the yavam engaged in levirate betrothal with one yevama, had sex with a second, and gave a bill of divorce to a third, the bill of divorce is effective and he is prohibited from marrying the relatives of the third yevama. And he concedes that levirate betrothal is effective after sex and a bill of divorce, such that if he gave a bill of divorce to one woman, had sex with a second, and performed levirate betrothal with a third, the levirate betrothal is effective and the third woman requires a bill of divorce.