What was Moshe's question? We often find multiple warnings!
Rashi: He claimed that it was unnecessary to warn Yisrael not to ascend Har Sinai, since they had been already prohibited from doing so since three days. 1
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 (21), Hadar Zekenim (21), Rosh (21): Moshe had not warned Yisrael about the Mitzvah of Hagbalah, only about separating [from their wives]. Hashem said that it is more important to warn about Hagbalah, which is punishable by death. Moshe answered, you warned us, including me and Aharon; we will be with them 2 , and warn them not to ascend! 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: Moshe asked, am I included in the Am, who are forbidden to ascend the mountain?
Divrei Eliyahu: Moshe said, you warned us, including me and Aharon, not to ascend. Surely the people will not ascend! Hashem responded, you and Aharon will ascend