Shevuos Chapter 3, Mishnah 7. This mishnah continues the subject of the previous one - layered oaths, that is, a person who swears and then swears again about the very same thing.
The wording of the mishnah: "Shevuah shelo ochal kikar zu, shevuah shelo ochlenah, u'shevuah shelo ochlenah, va'achalah - eino chayav ela achas" - even though he uttered the language of an oath three times, he is liable for only one oath.
The difference between the two formulations:
"Shevuah shelo ochal kikar zu" (an oath that I will not eat from this loaf) - even if he ate only a kezayis of the loaf, he has already become liable for his oath.
"Shevuah shelo ochlenah" (an oath that I will not eat it) - he is liable only if he ate the entire loaf.
Therefore, in our case: once he first swore "that I will not eat from this loaf," he has already forbidden even a kezayis of it to himself. When he went on and swore "that I will not eat it" - which refers to eating the entire loaf - the second oath cannot take effect, since he cannot begin eating the whole of it when even the first kezayis is already forbidden to him.
But if the order were reversed - that he first swore "an oath that I will not eat it," meaning that he will not eat the entire loaf, and afterwards added "an oath that I will not eat from this loaf," which means that he will not eat even a kezayis of it - the second oath does take effect, because it forbids more than the first oath forbade.
Why did the mishnah mention a third oath?
Seemingly this third statement accomplishes nothing at all. However, even though it does not take effect now, it is not wasted and it is not a false oath. For if he goes to a sage and requests annulment of his oath and receives a release - just as one requests annulment of vows, so too one requests annulment of oaths - and the first oath is annulled, the additional oath will automatically take effect, and eating the loaf will still be forbidden to him by its force.
"Zo hi shevuas bituy":
The mishnah concludes by setting out the penalties for the oaths discussed until now, which are called oaths of expression: "Zo hi shevuas bituy, shechayav al zedonah makos ve'al shigegasah korban oleh v'yored":
For its intentional violation - lashes: one who transgresses his oath intentionally receives lashes.
For its unintentional violation - a korban oleh v'yored: one who transgresses unintentionally brings an oleh v'yored offering, a korban that varies according to the financial situation of the one bringing it.
This is in contrast to a false oath: for that too one is liable to lashes, but unlike an oath of expression - for its unintentional violation he is exempt, and there is no korban for it.
In summary: In this Mishnah we learned that an oath does not take effect upon an existing oath, and therefore one who swears several times about the same matter is liable only once, unless the later oath added a prohibition beyond the earlier one. We also learned that an oath which does not take effect is not entirely void, since it comes into force if the first oath is annulled, and we clarified the penalties for a shevuas bittui (an oath of utterance) - lashes for an intentional violation and a variable offering (korban oleh v'yored) for an unintentional one - as opposed to a false oath (shevuas shav), for which there is no obligation to bring an offering when it was violated unintentionally.