Talmud - Nedarim 17a נדרים י״ז א׳

However, the verse states: “Or if anyone swears clearly with his lips to do evil, or to do good” (Leviticus 5:4). From the juxtaposition of evil and good it is derived that just as the doing of good, which is interpreted as obligating himself to take a positive action, is referring to a permitted activity, e.g., to eat, so too, the doing of evil, which is interpreted as prohibiting himself from something, refers only to that which is permitted, e.g., not to eat. This excludes one who takes an oath to nullify a mitzva and does not nullify it; he is not liable for violating the oath, as the permission to nullify it is not in his power.

The Gemara answers that both verses are necessary. One verse: “To do evil or to do good,” which is stated in the context of the halakhot of offerings, is necessary to exempt him from bringing an offering for violating an oath, and one verse: “He shall not profane,” is necessary to exempt him from the prohibition for violating an oath.

MISHNA: There is a vow within a vow. It is possible to impose an additional prohibition, by means of a vow, on an item that is already forbidden by means of a vow. But there is no oath within an oath. If one takes an oath twice with regard to the same action, the second oath does not take effect. How so? If one said: I am hereby a nazirite if I eat, and then repeated: I am hereby a nazirite if I eat, and then he ate, he is obligated to observe naziriteship for thirty days for each and every one of the vows, as both vows took effect. However, if he said: I hereby take an oath that I will not eat, and repeated: I hereby take an oath that I will not eat, and then he ate, he is liable to bring an offering for only one violation of an oath.

GEMARA: Rav Huna said: They taught that there is a vow within a vow only where he said: I am hereby a nazirite today, and then he said: I am hereby a nazirite tomorrow. As since an additional day of naziriteship is added by the second vow, an additional thirty-day obligation of naziriteship takes effect upon the first term of naziriteship. However, if he said: I am hereby a nazirite today, and then repeated: I am hereby a nazirite today, a vow of naziriteship does not take effect upon a previous vow of naziriteship, and he must observe only one term of naziriteship. And Shmuel said that even if he said: I am hereby a nazirite today, I am hereby a nazirite today, a second vow of naziriteship takes effect with regard to him, as one can apply two obligations of naziriteship to himself one after the other.

The Gemara asks: And according to Rav Huna, instead of teaching that there is no oath within an oath, drawing a distinction between a vow and an oath, let the mishna teach a narrower distinction between different vows, stating that there is a case of a vow within a vow, and there is a case in which there is not a vow within a vow. How so? If one says: I am hereby a nazirite today, I am hereby a nazirite tomorrow, in this case there is a vow within a vow. However, if one says: I am hereby a nazirite today, and then again says: I am hereby a nazirite today,

Traduction : The William Davidson digital edition of the Koren Noé Talmud, with commentary by Rabbi Adin Even-Israel Steinsaltz.

Nedarim 17a examines why oaths to nullify a mitzva carry no liability, then explores how multiple vows can stack while repeated oaths cannot.