Rav vis a vis halitza pesula, חֲלִיצָה פְּסוּלָה TB Yevamot 26

 In the first two chapters of our massekhet we were dealing with the laws of yibum, levirate marriage, on a Torah level. We learned all about how a prohibited marriage, an ’erva ערוה, impacts yibum. The third chapter deals with these laws on a rabbinic level. On a rabbinic level the rabbis forbade a person to do yibum with the sister of a woman whom he has already perform the ceremony of halitza (אחות חלוצתו). Similarly, the rabbis forbad a person to do yibum with the sister of woman whom he is obligated to do yibum (אחות זקוקתו). Since he can’t marry or perform yibum with these two categories of women on a rabbinic level, if either category a women falls before him because she is the widow of his brother who died without children he must perform the ceremony of halitza. How is this halitza going to be viewed? It is viewed as a deficient or less-than-perfect halitza. The Gemara describes it as a halitza pesula, חֲלִיצָה פְּסוּלָה, and Rashi in his commentary describes it as a halitza garu’a, חֲלִיצָה גרועה. Rav and Shmuel understand how this deficient halitza plays out.

Daf TB Yevamot 26 begins with Rav. “Rava bar Rav Huna said that Rav said: In a case where three sisters who were married to three brothers became yevamot and happened before two remaining brothers who were their yevamin for levirate marriage, then this one brother performs ḥalitza with one of the women, and that brother performs ḥalitza with another one of the women. And the middle, i.e., third, sister requires ḥalitza with both of the brothers.

“Rabba said to them: From the fact that you say that the middle one requires ḥalitza with both of the brothers, it can be deduced that you hold that the levirate bond is substantial and creates a familial relationship, and similarly this ḥalitza performed by each of the brothers with the third sister is invalid ḥalitza. Since the third woman is the sister of a woman with whom he performed ḥalitza, it is not possible to consummate the levirate marriage with her. Therefore, the ḥalitza performed with her is not entirely valid and does not release her from the levirate bond between her and the two brothers. And there is a principle that invalid ḥalitza must be repeated by all of the brothers; each of them must perform an act of ḥalitza with this woman.” (Sefaria.org translation)

Here is an excellent visual diagram to help you understand Rav’s position. https://hadran.org.il/wp-content/uploads/2022/03/Yevamot-26.pdf