Commentary for Exodus 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 d'Rabbi Yishmael 19:24:1).

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

רד ועלית אתה ואהרון, "descend and then ascend, you and Aaron." Why did G'd say both לך and רד? The word לך meant that Moses was to move, the word רד that he was to descend; perhaps he had to descend as he had previously not stood on a place assigned to him on the Mountain but had stood on the peak. When Moses refused to move G'd may have told him to descend (from that peak) but to ascend again with Aaron. He should know that just as there was a limit to how far Aaron could ascend there would also be a limit to how far he, Moses, could ascend. This is implied when the Torah describes Aaron as ascending עמך"with you."