Shevuos 4:3 - 4:4

ובאו לבית דין והודו פטורים – and even if they denied [that they had testimony about so-and-so] outside of the Jewish court on each oath for no denial outside the Jewish court is considered denial.

כפרו בב"ד חייבין על כל אחת ואחת – as the Biblical verse states (Leviticus 5:5): “When he realizes his guilt in any of these matters,[he shall confess that wherein he has sinned],” to be liable for each and every one.

מה טעם – that they are not liable other than for one [sacrifice] even if they were silent and denied [their knowing anything] at the end, and we don’t say that a denial exists for all of them to make them liable for each and every one (i.e., testimony).

הואיל – but whereas if they denied in the Jewish court the first time, they would not be able to retract and confess, for they already stated that they don’t know any testimony about him, and furthermore, they do not retract and state [testimony]; therefore, even though that they didn’t deny [knowing anything about so-and-so] until the end, all of the oaths were abrogated, except for the first one. For he fact that they were silent initially is denial, they are not worthy to be made to take an oath and if it is not denial, they are sworn in as standing witnesses. And what is an oath upon an oath? By force, when it is written (in Leviticus 5:5): “in any of these matters” – to be liable for each and every one, referring to an oath taken outside the Jewish court and they denied it in the Jewish court as it is written [in Scripture] that he would administer an oath to them when they would come to the Jewish court and they would testify and they would not leave in idleness, and it is appropriate for them to divide if they denied there at first, it would be appropriate to retract and to administer to them an oath.

כפרו שניהם כאחת – in an interval equivalent to the time of speaking (“Greetings to you, My teacher.”).

בזה אחר זה – that there was between [the testimony of] this one and [the testimony of] that one more than an interval equivalent to the time of speaking.

הראשון חייב – but the second is exempt, for since the first one denied, the second one is furthermore not appropriate to testify because he is an individual (and testimony requires two or more individuals).

שתיהן חייבות – in the Gemara (Tractate Shevuot 32b-33a) raises the objection – why is the first [set of witnesses] liable, for the second set [of witnesses] exists? For what have the first [set of witnesses] lost through their denial? And it answers that that the our Mishnah is speaking about q case where the witnesses of the second group were related through their wives (see Rashi’s comment: they married two sisters and are not valid as a singular testimony, and they were not valid to testify when the first set [of witnesses] denied [that they knew anything to testify] and their wives were on their deathbeds, for you might have thought that we hold that most people on their deathbeds will die, it was for them as if they [i.e., their wives) had died and the first set [of witnesses] were exempt but the second set exists. It comes to tell us now however that he is not dead. But it is found that only the first set [of witnesses] alone was there at the time of the denial, and therefore, they are liable.