Whenever there is a case of an actual mamzer, it is very difficult for the Jewish community, but especially for the mamzer himself or herself. A mamzer cannot marry within the Jewish community, and that makes life very difficult for this person.

The way a beis din deals with this is generally to find a way to invalidate the original marriage. That means when the mother had her child with another man other than her husband, she would not have been married, retroactively, thus her extramarital affair was not an extramarital affair and the child from the non-affair loses the mamzer status.

The problem is that this method does not always work. Sometimes, many times, they just cannot find a way to retroactively invalidate the marriage.

Thankfully, this troubling situation doesn't happen all too often.

Sadly, it has just happened.

The story began 45 years ago with a marriage in France. The young couple married, in both religious and civil ceremonies, and over time had two children. In 1992 the couple decided to divorce, and did so. Shockingly, Mrs revealed at the gett ceremony that she had another child, a daughter, born from an affair she had had with another man at a period in which she and her husband had been separated.

MAMZER ALERT!

I am not quite sure how this works because sometimes we say we believe a person about their own infidelities in regards to how it effects them but we do not believe them regarding the effect it has on other people, such as their children. This is called "palginan neemanus" - we split the believability of the person, only believing him/her in part.

Back to France..

Daughter now wants to get married. As a regular Jew to another Jew.

The beis din went researching the marriage of her parents, inspecting the ksuba and also interrogating people who were at the wedding and knew the couple.

Interestingly, they discovered problems with the original ksuba. For example, one of the names of the two witnesses was the same as the name of the father of Mrs. That could be an interesting coincidence,  or maybe her father acted as one of her witnesses.

The beis din decided they had enough to invalidate the original wedding, but sent the case to the Chief Rabbi of Israel, Rav Yitzchak Yosef, for his consideration. The case was given to the beis din of Haifa to work on and they decided Daughter is not a mamzeres, but for a completely different reason.

The beis din decided, base don piskei halacha of Rav Moshe Feinstein and Rav Ovadya Yosef, and others, that the concept of "double rov" applies. Basically, in such a case where it is not known the identity of the biological father, we attribute his identity to whatever the identity of the majority of the city is. That would be a problem in Jerusalem of Bet Shemesh or many other places in Israel, but in France it means the beis din is considering the father a non-Jew. If Mrs had an affair with a non-Jew, Daughter is not a mamzeres. This, in addition to the problem with the name of the witness, is enough for them to recommend that she be allowed to marry as a regular Jewess.

Rav Yosef confirmed this opinion and the beis din in France took it under consideration and decided that she is not a mamzeres and can marry as a regular Jewess.

sources: Kipa and Kikar