If the same 2 witnesses disqualify further sets of witnesses by saying "but you were with us - far away - when the crime happened" they are not put to death.
In the original:
בָּאוּ אֲחֵרִים וְהִזִּימוּם, בָּאוּ אֲחֵרִים וְהִזִּימוּם, אֲפִלּוּ מֵאָה, כֻּלָּם יֵהָרֵגוּ. רַבִּי יְהוּדָה אוֹמֵר, אִסְטָסִית הִיא זוֹ, וְאֵינָהּ נֶהֱרֶגֶת אֶלָּא כַת הָרִאשׁוֹנָה בִלְבָד :
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.”
What puzzles me is why Rabbi Yehuda is worried that it's a conspiracy.
Surely the disqualifying witnesses aren't believed without being cross-examined.
If they get through the rather grueling cross-examination needed in capital offenses - as described in the 5th chapter of Sanhedrin - then why aren't they believed for subsequent sets of witnesses?
Is this some type of Gezarat HaKatuv (i.e. does Rabbi Yehuda have a Pasuk or Mesora for this)? Or are these disqualifying witnesses not cross-examined? (Since the cross-examination is not mentioned anywhere.)
I can't find anybody who discusses this.