Gemara

51b - Mishnah: They would say 'Our ancestors faced the sun and bowed to it...and us, our eyes are to Hashem';

R. Yehudah says, they would double (the reference to Hash-m) and say 'We are to Hash-m, and to Hash-m are our eyes.'

53b - Question: But R. Zeira taught that one who says 'Shma, Shma' is like one who says 'Modim, Modim' (we silence him, for this looks like allegiance to two Powers)!

Answer: Rather, they said 'We thank (Bach - bow to) Hash-m, and our eyes hope to Hash-m.'

Berachos 33b - R. Zeira: Saying 'Shma, Shma' is like saying 'Modim, Modim'.

Question (Beraisa): It is despicable to read Shma and double it.

It does not say that we silence him!

Answer: It depends on whether he repeats each word, or repeats the entire verse. (In one case we silence him; the other is merely despicable.)

Question (Rav Papa): Perhaps he didn't have intent the first time, therefore he repeats it with intent!

Answer (Abaye): Does he treat Hash-m as a peer, and repeats himself and shows that he spoke to Him without intent?! He should be smacked until he has intent!

Rishonim

Rif (Berachos 23b): One who says 'Shma, Shma' is like one who says 'Modim, Modim'. This is only if he repeats each word. If he repeats a verse of Kri'as Shma it is despicable, but we do not silence him.

R. Yonah (DH v'Chol): The Rif explains that if he repeats each word it looks like he accepts two Powers, but not if he repeats the entire verse. Rashi explains oppositely; if he repeats each word it merely looks like he lacked intent the first time.

Question: We repeat the entire verse in Selichos and at the end of Yom Kipur (even the Rif agrees that this is despicable)!

Answer #1 (R. Yonah ibid.): Since it is an old custom and the entire Tzibur says it, it is clear that it is not acceptance of two Powers.

Answer #2 (Tosfos Berachos 34a DH Amar Pesuka): Perhaps we are concerned only for one who says Shma in the proper time and accepts Ol Malchus Shomayim. Nevertheless, it is good not to repeat it. It is no problem to say "Hash-m Hu ha'Elokim" seven times on Yom Kipur and Hoshanah Rabah; we praise Hash-m, Who lives above the seven Reki'im. Even the verse repeats this (twice).

Rambam (Hilchos Kri'as Shma 2:11): If one reads the verse (of Shma) and repeats it, this is despicable. If he repeats each word, e.g. 'Shma Shma', we silence him.

Kesef Mishneh: R. Mano'ach says that it is likewise despicable to repeat the entire Parashah. Others say that (if one did not intend the first time) it is better to repeat it and fulfill the Mitzvah properly, even if it appears despicable.

Lechem Mishneh: (Usually) when the Gemara answers 'This is not difficult. Here..., and there...', 'here' refers to the latter teaching. This supports Rashi, who says that the Beraisa discusses repeating each word. On the other hand, R. Zeira discusses 'Shma, Shma', which connotes repeating each word. This is like the Rambam.

The Rosh (Berachos 5:16) brings the Gemara in Berachos.

Rosh (ibid.): Rashi (DH Amar) says that we silence one who repeats the entire verse, for it looks like he accepts two Powers. According to this we should silence people who say "Shma Yisrael..." two or three times at the end of Yom Kipur. We are not concerned for repeating "Hash-m Hu ha'Elokim", for the verse itself repeats this. The Rif explains that we silence one who repeats each word.

Poskim

Shulchan Aruch (OC 61:9): It is forbidden to repeat Shma, whether he repeats each word or the entire verse.

Beis Yosef (DH Kosav ha'Kolbo): We are concerned for repeating only the first verse, not for later verses.

Mishnah Berurah (24): This is unlike the Rif (who says that it is despicable to repeat a verse of Kri'as Shma).

Sefer Chasidim (18): If one read the first verse without intent, he should read it again (quietly), for (if he repeats it out loud) it looks like he accepts two Powers.

Note: We added the words in parentheses to fit the Taz' understanding of Sefer Chasidim. Obviously, some correction to the text is necessary. The Perush to Sefer Chasidim brings another version which reads 'he should not read it again, for it looks like he accepts two Powers.'

Rebuttal (Taz 3): Rav Papa asked that perhaps one repeats because he did not intend the first time. Abaye did not answer that if so he should read again quietly! This shows that it is forbidden to repeat it even quietly.

Mishnah Berurah (22): If one read the first verse without intent, he should read it again quietly. If no one will hear him, he may read it out loud. If he intended the first time, it is forbidden even quietly. It is improper only if there is no pause in between, but if one waits in between it is fine.

Magen Avraham (7): Divrei Chamudos (Berachos 5:48) condemns the custom to repeat Shma in Selichos every time we say "va'Ya'avor..." The Shlah defends this. This seems correct, for there is a big separation in between.

Tur: The Yerushalmi says that the Isur is only b'Tzibur, but the Bavli does not distinguish.

Rebuttal (Bach DH veha'Omer): This is wrong. The Bavli discusses silencing him, i.e. b'Tzibur, when there are others to silence him. When no one hears him it is permitted, unlike the Tur and Shulchan Aruch.

Kaf ha'Chayim (34): We follow the Shulchan Aruch. However, if one read the first verse without intent, he reads it again quietly. If no one will hear him, reading out loud is like reading quietly.

Shulchan Aruch (10): One who reads Kri'as Shma Al ha'Mitah is allowed to repeat the entire Parashah.

Source (Gra DH keshe'Korei): The Yerushalmi (Berachos 6a) says that R. Shmuel bar Nachmani would repeat Shma until he fell asleep. SMaK explains that it is permitted to repeat the entire Parashah. BaHaG says that the Heter is only for Kri'as Shma Al ha'Mitah.

Mishnah Berurah (25): One may repeat the Parashah during Kri'as Shma Al ha'Mitah, in order to sleep amidst Kri'as Shma for Shmirah. If one repeats all the Parashiyos of Shma it is always permitted.

Shulchan Aruch (ibid.): Some say that one should not repeat the first verse.

Source (Beis Yosef DH v'Im): Mahari Avuhav says that one should not repeat the entire Parashah. R. Shmuel bar Nachmani would repeat Shma until he fell asleep, but perhaps he repeated from the second verse onwards.