http://revach.net/article.php?id=4357

Section: Parshas HaShavua   Category: Quick Vort
Parshas Yisro: Rav Levi Yitzchok of Berditchev - Moshe Open Your Eyes!
Before the Shloshes Yimei Hagbalah, Hashem told Moshe to tell Bnei Yisroel that they may not go on to Har Sinai or else they will die.  Then again on the day of Matan Torah, Hashem tells Moshe Rabbeinu to warn them not to go on to Har Sinai.  Moshe answers Hashem, "Lo Yuchal HaAm Laalos El Har Sinai Ki Ata HaEidosa Banu", the nation cannot ascend Har Sinai because You already warned them (Yisro 19:23).  Hashem then tells Moshe "Lech Reid", go down and warn them again.  Rashi says we see from here that you must warn someone twice.  Once to instruct him not to do something and then again Bshaas Maaseh, at the moment of truth, you warn him again.

Rav Levi Yitzchok of Berditchev in Kedushas Levi explains this slightly differently.  He says when Hashem told Moshe to tell them again, Moshe asked what for, since a person can't possibly violate the word of Hashem.  "V'lo Yuchal... Ki Ata HaEidosa", it is impossible that someone would fathom taking action that the Melech Malchei HaMelachim prohibited.  To this Hashem told Moshe, "Lech Reid" go down from your high level and understand the mentality of the nation.  To you this may be impossible, because you are Kadosh.  However you will be quite surprised at the capacity and audacity of mankind to disobey even the word of the Melech Yotzeir HaKol Ribon Kol Olamim, and you better warn them again!