What was Hashem's reply?
Rashi: Hashem answered that Moshe should descend 1 and warn the people again, due to the principle that one is obligated to warn the people when the prohibition is issued and again when the matter falls due.
Rashbam (in Pasuk 22): He now instructed Moshe to descend and to ascend once again together with Aharon; and as for the people, they were forbidden to break their ranks in order to ascend, but if they only intended to move closer to the mountain.
Da'as Zekenim (21), Hadar Zekenim (21), Rosh (21): Hashem responded, you and Aharon will ascend more than them; you will not be with them to warn them then, so warn them now.
Moshav Zekenim (23): You and Aharon will ascend, and they will be left like a flock without a shepherd.
Moshe was on such a high level that if Hashem issued a command, it was inconceivable to transgress it, in which case it was not necessary to repeat it. Therefore Hashem said to him "Lech Reid" - Come down from your high level! (See Rashi, Shemos 32:7). Much in the same way as the Gemara in Shabbos (20a) says about the Kohanim 'Kohanim Zerizim Heim' (Shabbos, 20a). The people however, are not on that level and require a second warning. (EC)
If Moshe ascended Har Sinai before the Aseres ha'Dibros were given, and the people did not move from their place, where were Aharon and the 'Kohanim'?
Rashi: The Kohanim accompanied Moshe part of the way and set up their camp, and Aharon accompanied Moshe still further, setting up camp somewhere in between the Kohanim and Moshe. 1
Seforno: Hashem instructed Moshe to descend and to remain with the people whilst the Aseres ha'Dibros were being said; and it was only after the Aseres ha'Dibros that He ordered Moshe to ascend with Aharon, and the Kohanim and the people to remain in their places. 2
The problem with this is that when Moshe eventually returned holding the Luchos, Aharon was in the camp, together with the people and it was Yehoshua, who had accompanied his Rebbe to the outskirts of the camp, who was encamped on his own? Moreover, the Pasuk implies that Aharon accompanied Moshe to wherever it was that he was going?