The Rambam, of blessed memory, wrote at the end of chapter 9: If one witness says, "This woman is betrothed," and she says, "I was not betrothed" – she is permitted. If one says she is betrothed and one says she is not betrothed – she shall not marry, but if she married, she does not go out, for she says, "I was not betrothed." Thus far his words. And behold, that which he wrote regarding her contradicting the witness, that she is permitted, and it is clear from his words that she is permitted even initially – this is a decided law throughout the Shas, that one witness in the face of contradiction is nothing, and therefore even if the witness says she was betrothed in the presence of two witnesses – it is nothing when she contradicts him. For if the witness were to say she was betrothed only in his presence alone, then even without the reason of contradiction it would be nothing, as the Rambam holds that one who betroths with one witness, we do not concern ourselves with his betrothal. However, that which he wrote, that if there is also a witness who contradicts him, she shall not marry initially, requires a reason; for if there is a witness who says like her, is it worse? And the commentators wrote that the Rambam holds it is indeed worse, for when there is a witness contradicting the witness, she is emboldened to say, "I was not betrothed," and therefore initially she shall not marry. And this is what the Sages, of blessed memory, said: One witness says she was betrothed and one witness says she was not betrothed – she shall not marry, but if she married, she does not go out. And this deals with a case where she also says, "I was not betrothed," for if she says, "I was betrothed," it is obvious that she has made herself a forbidden piece, and even if she says she has a doubt whether she was betrothed or not, and even if she married she goes out, for she concerns herself with the doubt of being betrothed. Therefore, even if she also says she was not betrothed, and were it not for the witness who says like her she would have been permitted initially, yet through the witness it is worse, as has been explained. And according to this, it must be said that this is specifically when the witness said first that she was not betrothed and afterwards she spoke like him; but if she said first that she was not betrothed, she was permitted immediately, and the reason of being emboldened is not relevant since she did not know there was a witness who says like her. And according to this, a new law emerges for us: that which we established that one witness in the face of contradiction is nothing, this is when a contradicting witness did not also precede the witness; but when a witness preceded – one must be concerned initially.
כתב הרמב"ם ז"ל בספ"ט: אמר עד אחד מקודשת היא זו, והיא אומרת לא נתקדשתי – הרי זו מותרת אחד אומר מקודשת ואחד אומר אינה מקודשת – לא תנשא ואם נשאת לא תצא, שהרי היא אומרת לא נתקדשתי, עכ"ל. והנה מ"ש במכחשת את העד שמותרת, ומבואר מדבריו שגם לכתחלה מותרת – זהו הלכה פסוקה בכל הש"ס דעד אחד בהכחשה לאו כלום הוא [ס"ה:], ולכן אפילו אם העד אומר שנתקדשה בפני שני עדים – לאו כלום הוא כשמכחישתו, דאם העד היה אומר שרק בפניו לבד נתקדשה הרי גם בלא טעם הכחשה אין זה כלום, דהרמב"ם ס"ל דהמקדש בעד אחד אין חוששין לקדושיו, אבל מ"ש דאם יש גם עד המכחישו ה"ז לא תנשא לכתחלה צריך טעם, וכי אם יש עד האומר כמותה מיגרע גרע? וכתבו המפרשים דס"ל להרמב"ם דוודאי מיגרע גרע, דכשיש עד המכחיש להעד מעיזה פניה לאמר לא נתקדשתי, ולכן לכתחלה לא תנשא, וזהו שאמרו חז"ל [כתובות כ"ג.] עד אומר נתקדשה ועד אומר לא נתקדשה ה"ז לא תנשא ואם נשאת לא תצא, ומיירי שגם היא אומרת לא נתקדשתי, דאם היא אומרת נתקדשתי פשיטא דשויא נפשה חתיכה דאיסורא ואפילו אומרת שיש ספק אצלה אם נתקדשה אם לא, ואפילו אם נשאת תצא שהרי חוששת לעצמה בספק מקודשת [הה"מ], ולכן אפילו גם היא אומרת שלא נתקדשה ואלמלי העד שאומר כמותה היתה מותרת לכתחלה, אבל ע"י העד גרע יותר כמ"ש, ולפ"ז צ"ל דדווקא כשהעד אמר תחלה שלא נתקדשה ואח"כ אמרה כמותו, אבל אם היא אמרה תחלה שלא נתקדשה הרי הותרה מיד, וגם לא שייך הטעם דמעיזה שהרי לא ידעה שיש עד האומר כמותה, ולפ"ז יצא לנו דין חדש דהא דקיי"ל עד אחד בהכחשה לאו כלום הוא זהו כשלא קדמו עד המכחיש ג"כ להעד, אבל כשקדמו עד – צריך לחשוש לכתחלה.