The Gemara in Sanhedrin 59a says that any mitzvah given to Benei Noach and repeated at Har Sinai, generally speaking, was given to both groups. Thus, both groups are given the issur of Avodah Zarah, for instance.
The Gemara challenges this premise based on the fact that Priah U'Reviah and Bris Milah were given to Benei Noach. Why are they not chayiv in them as well? The Gemara answers that they were repeated to teach special laws.
Now, before Har Sinai, these laws were not repeated. Does this mean that Benei Noach, pre-Sinai, were chayiv in Priah u'Reviah and Bris Milah? Do we say that it works retroactively, that since we see from the fact that it was repeated at Har Sinai that it's like they were never commanded in it, or were they commanded in it until it was retaught?