Study Guide Nedarim 17 Today's daf is sponsored by Diana Bloom in loving memory of her zeide, Israel Marmurek, on the occasion of his yahrzeit.  Today's daf is sponsored by Avi and Shelly Yonitzman in honor of their daughters' bat-mitzvah, Shira and Hallel. From where is it derived that an oath taken to not fulfill a mitzva is invalid? On Nedarim 16, it was derived from the word "devaro," his word, which refers to an oath about voluntary actions, excluding mitzvot. However, the Gemara brings a different source to teach the same law, "le'hara o le'heitiv" for good or for bad. From the juxtaposition of these words, they learn that it only refers to voluntary actions, as just a "to do good" which refers to positive actions (i.e. to eat) cannot include an oath that would cause one to not keep a mitzva (as inherently that is not good), also "to do bad" (i.e. to eat) would include an oath not to keep a mitzva. Why are both derivations necessary? Another stringency of vows over oaths is that if one makes a vow twice on the same thing, both vows are valid, whereas if one takes the same oath twice, the second oath is not valid. Rav Huna holds this is only true about a vow if they do not 100% overlap, such as, "I will be a nazir today" and "I will be a nazir tomorrow." Shmuel says it is true even if the second vow is 100% identical to the first, such as, "I will be a nazir today" and "I will be a nazir today." The Gemara raises difficulties with Rav Huna's opinion from four different sources (two from our Mishna). Three of the difficulties appear in this daf, one more in the continuation in Nedarim 18).