Nedarim 2:2-3

Nedarim 2:2

If a person says, “korban that I will not eat from you,” “korban that I will eat from you” or “not a korban that I will not eat from you,” it is permitted. If one says, “shevuah that I will not eat from you,” “shevuah that I will eat from you” or “no shevuah that I will not eat from you,” it is prohibited. In this way oaths are more stringent than vows. In other ways, vows are more stringent than oaths. For example, if one said, “konam the succah that I build,” “the lulav that I take” or “the tefillin that I wear,” these things are prohibited with vows and permitted with oaths. This is because one cannot take an oath to violate a mitzvah.

Nedarim 2:3

There is such a thing as a vow within a vow but there is no such thing as an oath within an oath. For example, if a person said, “I am a nazir if I eat, I am a nazir if I eat” and then he ate, he must fulfill each vow (i.e., a 30-day nazirite period for each statement). If he said, “shevuah that I will not eat, shevuah that I will not eat” and then he ate, he need only observe one oath.