When one witness testifies that a husband died and the wife married based on this, a second witness testifying that he did not die does not require her to leave her new husband, since she remains in the status of permission initially granted to her.
מַתְנִי׳ עֵד אוֹמֵר ״מֵת״, וְנִשֵּׂאת, וּבָא אֶחָד וְאָמַר ״לֹא מֵת״ — הֲרֵי זוֹ לֹא תֵּצֵא. עֵד אוֹמֵר ״מֵת״, וּשְׁנַיִם אוֹמְרִים ״לֹא מֵת״ — אַף עַל פִּי שֶׁנִּשֵּׂאת, תֵּצֵא. שְׁנַיִם אוֹמְרִים ״מֵת״, וְעֵד אוֹמֵר ״לֹא מֵת״ — אַף עַל פִּי שֶׁלֹּא נִשֵּׂאת, תִּנָּשֵׂא. גְּמָ׳ טַעְמָא דְּנִשֵּׂאת, הָא לֹא נִשֵּׂאת — לֹא תִּנָּשֵׂא. וְהָאָמַר עוּלָּא: כׇּל מָקוֹם שֶׁהֶאֱמִינָה תּוֹרָה עֵד אֶחָד — הֲרֵי כָּאן שְׁנַיִם, וְאֵין דְּבָרָיו שֶׁל אֶחָד בִּמְקוֹם שְׁנַיִם. הָכִי קָאָמַר: עֵד אֶחָד אוֹמֵר ״מֵת״, וְהִתִּירוּהָ לְהִנָּשֵׂא, וּבָא אֶחָד וְאָמַר ״לֹא מֵת״ — לֹא תֵּצֵא מֵהֶיתֵּירָהּ הָרִאשׁוֹן.
MISHNA: If one witness says: The man died, and the wife married based on this testimony, and one other witness came and said: He did not die, she need not leave her new husband due to this testimony. However, if one witness comes and says: The husband died, and two witnesses say: He did not die, then even though she married based on the first witness she must leave her new husband. If two witnesses say: He died, and one witness says: He did not die, the testimony of the two witnesses is accepted, and even though she did not yet marry, she may marry. GEMARA: The Gemara infers: The reason in the case of one witness contradicted by another witness is that she already married; however, if she did not yet marry and a second witness comes in the meantime and contradicts the statement of the first one, she may not marry. The Gemara asks: But didn’t Ulla say: Wherever you find that the Torah relies on one witness, his testimony is considered complete proof, as though there are two witnesses present here? If so, the witness who comes and testifies to the opposite is only one witness, and the statement of one witness has no standing in a place where it is contradicted by two witnesses. Why, then, may she not remarry, even ab initio? The Gemara answers that this is what the mishna said: If one witness says: He died, and they permitted her to marry based on his testimony, and one other witness later came and said: He did not die, she does not leave her initial, permitted state, i.e., the permission she was granted to remarry is still in force, and she may marry ab initio.