Rashi answers your question when he says on verse 24:
Go, descend: And warn them a second time. We admonish a person before
the act [he is to perform], and we admonish him again at the time of
the act [when it is to be performed]. [from Mechilta]
Moshe evidently thought that one warning was enough and the intent of his response was to understand the need for another warning. G-d taught him that you need two – one before and one at the time.
The רא"ש asks your question.
My translation and elucidation of the רא"ש:
Moshe wanted to know how far up the mountain he could go. Moshe said
as follows, “I thought that there is no need for this warning because
you commanded me and Aharon my brother to fence the mountain and make
it holy. If the people would break through, we would warn them because
we thought that we will be with them.”
Then Hashem revealed to Moshe that after going down to warn the
people, Moshe and Aharon were to ascend and that, the people, Moshe
and Aharon would each be in their specific places. Therefore Moshe would
not be with the people to protect them and the second warning was
The דעת זקנים says
The first time, Moshe did not warn the people that the punishment for
going beyond their limit on the mountain would be death. So Hashem
asked Moshe to tell them of the punishment. Moshe answered that he
expected to be with the people to guard them to ensure that they would
not go up. To this point Hashem answered that Moshe and Aharon would
not be with the people (as explained by the רא"ש).
The explanation of both the רא"ש and the דעת זקנים fits well with the next possuk which says that Moshe and Aharon were to go up on the mountain.
See also the explanation of the אור החיים.