פּירוש אויף שמות 19:24

לך רד GO, GO DOWN and warn them a second time. God said this, because the rule is that one admonishes a person before an action is to be done and then one again admonishes him at the very moment when the action is to be done (cf. (Mekhilta).

ויאמר אליו ה' לך רד, Hashem said to him: “go and descend, etc.” If you were to ask why was there any need for three separate warnings (verse 12, verse 21 and this verse)? The warning in verse 23 refers to the warning in verse 21 where G-d had warned the people not to approach the mountain in order not to become a victim of destruction by trying to have a visual image of G-d. Moses had assumed that the very fact that they had been warned would be equivalent to the people’s inability to violate G-d’s commandment. G-d had already said in verse 12 that the people would be immediately killed if they were to try and ascend the Mountain or even its lower edge. Surely they could exercise control over their feet! Concerning their having control over their eyes, not to try and see the glory of G-d was a much more difficult task. This is why the warning had to be repeated. They would think that it was a good deed to try and catch a glimpse of their King! Moses had misinterpreted the prohibition as applying only to a physical ascent of the Mountain. This is why G-d had to add that not only ascent was potentially fatal, but even touching the base of the Mountain and even trying to catch a glimpse of G-d’s Majesty.

לך רד, now, when I am speaking with you, be with them at the bottom of the mountain.

ועלית אתה ואהרן עמך והכהנים AND THOU SHALT COME UP, THOU AND AARON WITH THEE AND THE PRIESTS — One might think that they (the priests) shall also be with thee! Therefore it states “and thou shalt come up”. Consequently you must now admit that this is what God spake to him: thou hast a designated place for thyself, and Aaron a designated place for himself and they a designated place for themselves — Moses approached closer than Aaron and Aaron closer than the priests — but let the people under no circumstances break down their position to go up to the Lord (Mekhilta).