Commentary for Makkot 1:5
בָּאוּ אֲחֵרִים וְהִזִּימוּם, בָּאוּ אֲחֵרִים וְהִזִּימוּם, אֲפִלּוּ מֵאָה, כֻּלָּם יֵהָרֵגוּ. רַבִּי יְהוּדָה אוֹמֵר, אִסְטָסִית הִיא זוֹ, וְאֵינָהּ נֶהֱרֶגֶת אֶלָּא כַת הָרִאשׁוֹנָה בִלְבָד:
If others came and they (the refuters) rendered them zomemin; if others came, and they rendered them zomemin — even if they were a hundred [sets of witnesses, giving the same testimony one after the other, and all refuted by the same set], they are all killed. R. Yehudah says: "This (refuting) set is istasith" [perverse and devious, having taken counsel between themselves to refute whoever comes to give this testimony. Another interpretation: "Is this (refuting set a vat of) isatis" (a dye), (which dyes all who touch it)]! And only the first set of witnesses is killed. [R. Yehudah holds that after the first set is refuted, the succeeding set is not accepted; and if they do testify and are refuted, they are not killed, not satisfying: "Then you shall do to him, etc.", for he (the adjudged) is not killed by their testimony. The halachah is not in accordance with R. Yehudah.]
Bartenura on Mishnah Makkot
English Explanation of Mishnah Makkot
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.