Gemara

73a (Mishnah): Mid'Oraisa, a Shetuki (one who recognizes his mother but not his father) is Kosher. It says "Lo Yavo Mamzer (bi'Khal Hash-m)." A definite Mamzer is forbidden, a Safek Mamzer is permitted. A Mamzer may not marry a Vadai member of Kehal Hash-m (one of Kosher lineage), but he may marry a Safek.

74a (Mishnah): All who may not marry into the Kahal, are permitted to each other;

R. Yehudah forbids.

R. Elazar says, Vadai (definite) Mamzerim may marry Vadai Mamzerim. Safek Mamzerim may not marry Vadai or Safek Mamzerim.

75a (Rav Yehudah citing Rav): The Halachah follows R. Elazar.

Rishonim

Rambam (Hilchos Isurei Bi'ah 18:17): The Isur of every Safek is mid'Rabanan.

Rashba (Kidushin 73a DH Mamzer): In a Teshuvah, the Rambam learned from Safek Mamzer that 'we are stringent about a Safek mid'Oraisa' is only mid'Rabanan.

Ran (Kidushin 15b DH Garsinan): The Ramban learns from Safek Mamzer that the Torah permits Sefekos. This is wrong. If so, why did Rava need to elaborate?! Let him say that Sefekos are permitted! Also, why did he discuss particularly Safek Mamzer? The Gemara says that R. Yosi uses one "Kahal" to permit a Mamzer to marry a Safek Mamzer, and another to permit a Safek Mamzer to marry a Yisrael.

Merumei Sadeh (Kidushin 73a DH Mamzer): The Rambam does not learn from the Heter for a Mamzer to marry a Safek, for perhaps we learn this from an extra "Kahal". Rather, he learns from the Heter of a Safek Mamzer (to marry Kahal), for there is no extra 'Mamzer'. Presumably, Mamzer is like Kahal; only a Vadai is forbidden. In the Hava Amina, R. Yosi expounded the Heter of a Safek Mamzer from an extra "Kahal". Rava concluded that he is permitted without "Kahal". He uses "Kahal" to permit a Shtuki to Kehunah. Yevamos 88b learns from "Dafno" that (mid'Oraisa) we are stringent about a Kohen (who married a woman who is Safek forbidden) than Yisre'elim. I.e. it is permitted for a Yisrael, and forbidden for Kohanim.

Question (Maharit 2 YD 1 DH va'Ani): How can the Ramban learn from Safek Mamzer? If we learn from there to all Isurim, why did the Gemara need to say that Chachamim were stringent about lineage? They are stringent about every Safek mid'Oraisa! Rather, they were stringent only for lineage. The Ramban holds that the Torah totally permitted Safek Mamzer, not merely due to Safek. He may marry a Mamzeres and Bas Yisrael together; we do not call this a contradiction. Therefore, we asked why Chachamim forbade.

Pri Chodosh (YD 110 Beis ha'Safek DH Od Kosav): We expounded three "Kahal"s to forbid. It is reasonable that the other two, which teach about Safek Mamzerim, also forbid. The Rashba holds that a Safek is always forbidden, so we needed to expound to permit a Safek. According to the Rambam, why didn't we expound to forbid a Safek Mamzer? It is difficult to say that the Rambam agrees that the a priori reasoning is that we are stringent about a Safek, and the verse of Mamzer teaches that we are lenient. If so, why were two verses needed? One might think to distinguish between contradictory leniencies done at once, and one after the other. However, the Torah does not. In either case, they are contradictory! If two people walked on different paths, one of which is Tamei, the Gemara says that we are lenient only if they ask separately, but not if they ask at once. Tosfos (Pesachim 10a DH b'Vas) says that mid'Oraisa we are lenient even when they asked together. We cannot learn from there to Safek Mamzer, for the paths involved two people, and Safek Tum'ah in Reshus ha'Rabim is Tahor. In Shevu'os 19a, we say that if one walked on both paths and later entered the Mikdash, he is liable. Tosfos (DH Holach) says that the paths are in Reshus ha'Rabim. Even though the Torah permits a Safek in Reshus ha'Rabim, it does not permit contradictory Sefekos. This is like the Maharit's defense of the Rambam. Alternatively, the Rambam holds that the Torah needed to permit the child of a woman who was Safek divorced, even though she had Chezkas Isur. Mid'Oraisa, the mother's Chazakah helps for the daughter to be lenient, and all the more so to be stringent. (One opinion is stringent about the daughter mid'Rabanan, even when her mother has Chezkas Heter.)

Ginas Veradim (Klal 7 DH v'Yesh Omrim): The Rambam uses the verse to permit marrying two Safek Mamzeros, even if surely one is a Mamzeres.

Shev Shemaitsa (1:1 DH v'Hikshu): If the Chidush is that we may be lenient about two contradictory Sefekos, why were two verses needed? The Pnei Yehoshua (73 DH Ela) says that the verse is needed for when a majority suggests that the Safek is really a Mamzer. I say that we follow the majority, even regarding Mamzer! The Gemara (Chulin 11a) relies on a majority to kill one who strikes his father, even though only Chazakah or a majority says that it is his father. If we do not rely on the majority and Chazakah for Mamzer, there is no Kahal Vadai! The Pri Chodosh does not discuss a proper Chazakah, rather, when it was weakened, e.g. through Safek divorce. Tosfos (Bava Basra 24a DH Leima) forbids Orlah in Chutz la'Aretz when there is a Rov, even though the Torah permitted Safek. Rather, R. Yosi requires two verses because he obligates Asham Taluy when there is only one piece, for he holds that Safek mid'Oraisa l'Chumra is mid'Oraisa.

Avnei Nezer (EH 18 3,4): It is difficult to say that the Rambam learns from a Mah Matzinu from Mamzer. Chachamim after the Gemara do not expound using the 13 Midos! Also, we should say that the Kahal that permits a Safek Mamzer, and the Kahal that permits a Mamzer to Safek Kahal, are Shenei Kesuvim, and we do not learn from them! Rather, the Rambam was bothered why we don't learn a smaller Chidush, that Beis Din need not reveal Safek Mamzerim, but one who knows about a Safek Mamzer must refrain. He concluded that this is because the Torah permits a Safek Isur.

Shev Shemaitsa (1:2): It seems that the Torah permits a Safek Mamzer when women gave birth and the babies became mixed. Even though one brings an Asham Taluy, the Torah permitted a Safek Mamzer!

Avnei Nezer (EH 18:19 and CM 129): The Torah permits Safek Mamzer even against a majority, just like Safek Orlah in Chutz la'Aretz (Kidushin 39a). Tosfos (Eruvin 46a) says that we are lenient about Safek Aveilus, even on the first day, which is mid'Oraisa. In Nazir (49a), there was a Hava Amina that a Kohen may not be Metamei for his father, for perhaps it is not his father. If so, he is not a Kohen! The Rambam says that Aveilus overrides Tum'ah. If we would consider it a Safek, he would be exempt from Aveilus and forbidden to be Mitamei. The Rashba said that the Gemara connotes that we permit Safek Mamzer only regarding an even Safek, when we know that his mother went to the Bo'el. If we do not know, the majority is Machshir the child! If so, why may a Shtuki marry a Mamzer?

Poskim

Shulchan Aruch (EH 4:24): A Mamzer may marry a Mamzeres if both of them are Vadai. If one is Vadai and the other is Safek, or even if both are Safek, they are forbidden, lest one of them is a Mamzer and the other is not.

Pischei Teshuvah (28): The Maharit (YD 2:1) permits a Safek to marry a Yisraelis and Mamzeres at once. Sefekos are like a Kahal unto themselves. They are not considered Kahal (Hash-m).