לך רד. וְהָעֵד בָּהֶם שֵׁנִית, שֶׁמְּזָרְזִין אֶת הָאָדָם קֹדֶם מַעֲשֶׂה וְחוֹזְרִין וּמְזָרְזִין אוֹתוֹ בִּשְׁעַת מַעֲשֶׂה (מכילתא): 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).
ועלית אתה ואהרן עמך והכהנים. יָכוֹל אַף הֵם עִמְּךָ, תַּ"לֹ וְעָלִיתָ אַתָּה, אֱמֹר מֵעַתָּה, אַתָּה מְחִצָּה לְעַצְמְךָ, וְאַהֲרֹן מְחִצָּה לְעַצְמוֹ, וְהַכֹּהֲנִים מְחִצָּה לְעַצְמָם – מֹשֶׁה נִגַּשׁ יוֹתֵר מֵאַהֲרֹן, וְאַהֲרֹן יוֹתֵר מִן הַכֹּהֲנִים, וְהָעָם כָּל עִקָּר אַל יֶהֶרְסוּ אֶת מַצָּבָם לַעֲלוֹת אֶל ה' (שם): 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).
פן יפרץ בם. אַעַ"פִּ שֶׁהוּא נָקוּד חֲטַף קָמָץ אֵינוֹ זָז מִגְּזֵרָתוֹ, כָּךְ דֶּרֶךְ כָּל תֵּבָה שֶׁנְּקֻדָּתָהּ מְלָאפוּם, כְּשֶׁהִיא סְמוּכָה בָאָה בְּמַקָּף, מִשְׁתַּנֶּה הַנִּקּוּד לַחֲטַף קָמָץ: פן יפרץ בם LEST HE BURST FORTH UPON THEM — Although it (the word יִפְרָץ) is punctuated with a Chataf Kametz (our Kamets Chataf — short Kametz), it has not departed from its normal grammatical form, for thus is the way of every word which has the vowel Melopum (our חולם) in its hit syllable, that being a closed syllable: when it comes with a Makkef (a hyphen) after it the vowelling is changed to a Chataf Kametz (our short Kametz).