פירוש על מכות 1:5

Even if there were a hundred. pairs that testified one after the other with one testimony and one pair zommemizes all of them, all of them are killed.

If other witnesses came, and they charged them [with perjury]: then [again] others came, and they [again] charged them [with perjury], even a hundred, they are all to be executed.
Rabbi Judah says: “This is a conspiracy and the first set alone is [to be] executed.”
Mishnah five deals with multiple sets of perjuring witnesses.
According to the opinion in the first section of the mishnah every set of witnesses that perjures themselves with regards to a certain crime can be executed for perjury (if the case was a capital case). If, for instance, Reuven and Shimon testify that Jacob committed a crime which entails the death penalty and then Levi and Judah come and state that at the time of the alleged crime Reuven and Shimon were with them and not at the scene of the crime, Reuven and Shimon are executed. If afterwards Zevulun and Issachar come and testify to the same crime and again Levi and Judah claim that they were with them at the time of the crime, then Zevulun and Issachar are to be executed as well. Levi and Judah can testify against as many witnesses as they want, and even one hundred witnesses, all saying the same thing, could be executed for perjury.
Rabbi Judah claims that in this scenario there is a conspiracy between the accused and those who contradict everyone else’s testimony, namely, Levi and Judah. While we do accept their testimony, only the first set of witnesses are executed.

This is a conspiracy. This [word istetit] means that this group has strayed and are on the wrong path, as they schemed between themselves to zommemize all the witnesses that come to testify in this case. Another explanation of "this is an istetit", is these witnesses are shooters of istis, that it colors all that touch it.

Only the first group is killed. Since Rabbi Yehudah holds that after the first group is zommemized, the testimony of a second group that comes is no longer accepted, and if [a second group does in fact] testify and is zommemized, they are not killed, since we don't apply [the verse] "do to them", since we would not kill based off their [the second group's] testimony. The halacha is not like Rabbi Yehudah.