Nedarim 18A

ותיובתא דרב הונא!

and this is a conclusive refutation of the statement of Rav Huna.

לא, לעולם ״הריני נזיר היום, הריני למחר״, ומאי ״עלתה לו״⁠ — לבר מההוא יומא יתירא. אי נמי, כגון שקיבל שתי נזירות בבת אחת.

The Talmud answers: No, actually the case is where he said: I am hereby a nazirite today, I am hereby a nazirite tomorrow. And what is the meaning of the statement: The second term of naziriteship is counted for him instead of the first? It is counted except for that additional day, which he still must observe. Alternatively, it may be a case where he accepted upon himself two periods of naziriteship simultaneously, i.e., he said: I am hereby a nazirite twice. Therefore, when the vow with regard to the first term of naziriteship is dissolved, the days he observed count entirely for his second term.

מתיב רב המנונא: ״נזיר להזיר״⁠ — מכאן שהנזירות חל על הנזירות. שיכול, והלא דין הוא: ומה שבועה חמורה ⁠— אין שבועה חלה על שבועה, נזירות קלה לא כל שכן ⁠— תלמוד לומר: נזיר להזיר, מכאן שהנזירות חלה על הנזירות.

Rav Hamnuna raised an additional objection against the opinion of Rav Huna from a baraita. It is stated in the verse: “A nazirite, to consecrate [nazir lehazzir]” (Numbers 6:2). From the use of similar, repetitive wording in the verse here it is derived that naziriteship takes effect upon a prior vow of naziriteship. As one might have thought that it could be derived through an a fortiori inference that naziriteship does not take effect, as follows: And just as with regard to an oath, which is more stringent, an oath does not take effect upon a prior oath, with regard to naziriteship, which is more lenient, all the more so is it not clear that it does not take effect where a vow of naziriteship was already in effect? Therefore, the verse states: “Nazir lehazzir.” From here it is derived that naziriteship takes effect upon a prior vow of naziriteship.