It is recommended that one read this post in order to understand some of the
issues discussed here.
According to Maimonides, the pre-Sinaitic laws ceased to exist after the Sinaitic Revelation and only the Torah's laws were applicable to Jews thereafter. In describing the mitzvos, the Talmud says that six-hundred and thirteen mitzvos were given to Moses at Mount Sinai, but makes no mention of those commandments given before the revelation at Sinai because those commandments became null and void with the giving of the complete Torah. Rabbi Eliyahu Mizrachi (1455-1526) argues that all of the previous commandments are still in effect, plus they were repeated at Mount Sinai in the context of the Torah. Even though the commandments given before Sinai are still in effect, they were reiterated again at Mount Sinai so that they could be included within the Torah. He writes that the commandments of procreation, circumcision on the eighth day, and of refraining from eating the sciatic nerve were all repeated again the Torah. Furthermore, the Mizrachi says that "These are the commandments that HaShem commanded Moses to the Israelites on Mount Sinai" includes the entire content of Genesis, Exodus, and Leviticus; so the entire Torah until then is doubly commanded.
This dispute between the Mizrachi and Maimonides regarding whether the pre-Sinaitic laws are still effect can be rooted in a dispute concerning whether one can hermeneutically or otherwise derive details of laws using pre-Sinaitic precedent. Rashi writes that one can justifiably learn that proper time for praying Mincha from a precedent set by Abraham. However, Rabbi Nosson ben Yechiel Ba'al HaAruch (1035-1106) argued on this presumption because one cannot gain knowledge of Torah laws using events which occurred before the giving of the Torah at Mount Sinai. Similarly, Tosafos write that one cannot learn from the seven-day mourning period over the death of Jacob that the entire seven days of mourning are biblical because one cannot apply actions from before the receiving of the Torah to the post-Sinaitic law. Therefore, Tosafos rule that seven days of mourning are only rabbinically required, but not biblically mandated. However, Rabbi Yitzchok Alfasi (1013-1103) writes that we learn from the seven days of mourning over Jacob that all seven days of mourning are biblically required. It serves to reason that Tosafos concurs with the Aruch that one cannot learn post-Sinaitic laws from laws before the giving of the Torah, while Rashi and Alfasi argue that one could. Rabbi Isser Zalman Meltzer (1870-1953) explains that although Maimonides rules in favor of the Alfasi in regard to laws of mourning, this is not a contradiction to his opinion that all pre-Sinaitic commandments became obsolete after the giving of the Torah. This is because the latter opinion does not mean that one cannot at all learn laws from before Sinai, rather it means that one can only learn the law itself from before Sinai, but not details of the law. Therefore, Maimonides learns the law of mourning itself from Sinai, but in regard to details of other laws, he would not deduce the law based on pre-Sinaitic precedent.
 Maimonides' Commentary to the Mishnah, Chullin 100b
 Makkos 23b
 Mizrachi to Genesis 1:1
 Genesis 1:28
 Genesis 17:12
 Genesis 32:33
 Procreation was repeated in Deuteronomy 5:26 (one's "tent" euphemistically refers to his wife), circumcision was repeated in Leviticus 12:3, and the prohibition of eating the sciatica is again repeated in Deuteronomy 12:23
 Leviticus 27:34
 Yoma 28b
 As quoted in the Mesoras HaShas to the standard Vilna Shas print of Yoma 28b
 Moed Koton 20a
 Genesis 50:10
 Brachos 10b in the pages of the Rif
 Some attempt to prove that Rashi learned that one cannot learn halacha from laws before Sinai because he asked (in his commentary to Genesis 1:1) why the Torah began describing the creation of the world instead of with the first commandment which was sanctifying the month (Exodus 12:1 ff). According to this understanding, Rashi learned that all laws given before Mount Sinai were obsolete so the Torah need not have recorded everything until Exodus 12:1. However, this proves nothing in this discussion because had Rashi understood that, then he would not have asked specifically that the Torah should start from Exodus 12:1 because the commandments described therein were also given before Mount Sinai.
 Even HaAzel to Maimonides' Laws of Mourning 5:2
 Like the Pnei Moshe to Jerusalemic Talmud's Moed Koton 3:5 explains