Based on the principle that one is obligated to warn the people when the prohibition is issued and again when the matter falls due, what was Moshe's question?
Rashbam: He was asking Hashem whether, having initially prohibited the people from ascending the mountain, He was now adding a prohibition to break their ranks and to move a little closer, even without ascending the mounting.
Da'as Zekenim, Hadar Zekenim, Rosh (all in Pasuk 21) and Divrei Eliyahu: Moshe 1 was saying that, since he and Aharon were included in the initial warning, and they would be in the camp together with the people, when the people saw that they are not ascending, they (the peopl will not ascend either. 2
Moshav Zekenim: Moshe was asking whether he was like the rest of the people, who were forbidden to ascend the mountain..
Rosh: Moshe asked in order to learn how far he will ascend, though his question assumed that he would be like everyone else.
Da'as Zekenim, Hadar Zekenim, Rosh, Ibid.: Da'as Zekenim, Hadar Zekenim, Rosh: Initially, Moshe had not warned Yisrael about the Mitzvah of Hagbalah, only about separating [from their wives], and when Hashem told him that it was more important to warn about Hagbalah, which is punishable by death, this is what he replied.
