Shevuos Chapter 4, Mishnah 3: the oath of testimony. We begin with the basic case from which the obligation to bring a korban is derived, and from there we move on to the laws of multiple oaths, both in beis din and outside it.
The case the Mishnah is discussing:
A person said to two men, potential witnesses: "Come and testify for me," and they answered, "shevuah she'ein anu yod'in lach edus" - "we swear that we have no testimony for you." Or they answered without any language of an oath - "we have no testimony for you" - and the claimant said to them, "I impose an oath upon you," and they answered "amen." In both of these ways the witnesses are obligated to bring a korban for having violated the oath of testimony.
They swore outside of beis din:
If the claimant imposed the oath upon them five times outside of beis din, and afterwards they came to beis din and admitted that they did in fact know testimony, they are exempt. Even though they denied knowing testimony outside of beis din, a denial made outside of beis din has no force, since the wording of the verse is "im lo yagid v'nasa avono" - "if he does not testify, he shall bear his sin," and the Gemara explains that this applies only in a place where giving testimony accomplishes something, that is, in beis din alone.
However, if after swearing five times outside of beis din they came to beis din and denied, meaning they denied in beis din itself that they know testimony, they are obligated for each and every one, for every single oath that they swore.
They swore in beis din:
If they did not swear outside of beis din but rather in beis din, and they denied each and every time, and even if they were silent and only denied at the very end, they are obligated for one oath alone.
Rabbi Shimon's reasoning:
What is the reason that there is an obligation for only one of the oaths? Because they can no longer go back and admit that they do know testimony. Their denial in beis din has the status of giving testimony, since they testified that they do not know any testimony; and we have a general rule that once a person has testified in beis din one way, he can no longer go back and testify a different way. Consequently, it is no longer in their power to become admitters afterwards, and the first denial is what stands.
In other words: once they denied after the first oath, all the oaths that follow it are meaningless. From the moment they said "we have no testimony for you," they can no longer retract, and therefore they were already disqualified from testifying after the first denial. It follows that the later oaths cannot take effect, since they are no longer able to testify at all.
And if they were silent and only denied at the end:
Even in a case where they did not deny until the very end, but rather swore five oaths and only afterward denied, either way they are not liable for each and every one but for one alone. For they remained silent after the first oath, and we must clarify how we relate to that silence:
If the silence is considered a denial: as has been explained, once a person has denied in beis din he can no longer retract and testify, and the subsequent oaths are irrelevant.
If the silence is not considered a denial: once the first oath has been sworn, an oath does not take effect on top of an oath unless there is a reason to administer it; and since an oath already rests upon him, there is no reason to administer an additional oath.
Only when the oaths were sworn outside of beis din, where the possibility exists that they will retract after each and every time, does each oath carry meaning and content. But in the case before us, where everything took place in beis din, the oaths after the first one have no significance.
In summary: we have learned the case of the oath of testimony and the obligation to bring a korban that comes with it; that a denial outside of beis din does not count, and therefore an admission in beis din exempts; that one who swore five times outside of beis din and then denied in beis din is liable for each and every oath; whereas one who swore in beis din is liable for only one, since after the first denial - which has the status of giving testimony - he no longer has the power to testify, and the subsequent oaths do not take effect.