They were already warned

This seems difficult to understand. Why would they need a second warning? A few posukim earlier, HASHEM told Moshe, “Warn them that whoever will touch the mountain will die.” If they were already told, why would they now need a warning?
What makes this question particularly troubling is that this generation understood the gravity of their actions. They lived through the Maakos and then Krias Yam Suf. They saw the glory and power of HASHEM. At this moment they were privy to a level of understanding that few humans ever achieved, which means that they not only understood that HASHEM didn’t want them going up to the actual mountain, they also understood that if they did, they would surely die.

Breaking into Fort Knox

Imagine we were to meet a man with an extraordinary desire for money. Nothing can stop him. He would steal, cheat, lie, swindle -- anything to make his fortune. But even he won’t be tempted to break into Fort Knox. The armed guards standing there, machine guns poised, are so threatening, and the danger so real that despite his great desire for money, the certainty of his death will prevent him from even thinking of breaking in. So too, this generation fully understood the consequences of moving forward. How would it be possible that they would still be tempted to go up the mountain? What possible reason would HASHEM have to send Moshe back down to warn them again?

Human comprised of distinct parts

The answer to this question seems to be based on a fundamental understanding of man.
To make man, HASHEM took opposites entities and molded them into one. Inside me is a part with deep-set needs to do that which is noble and proper -- and within me is a part that just doesn’t care. The part of me that only wants to that which is good is made up of pure intelligence — Sechel. The part of me that doesn’t care about anything other than my own needs is made up of base instincts and desires — the Nefesh Ha’Bahami.
My Sechel clearly recognizes HASHEM as the source of everything and only wishes to be close to my Creator. It actually hungers to be close to HASHEM and pulls me that way. My Nefesh Ha’Bahami can’t recognize or understand anything other than drives and desires, hungers and appetites. It can’t see HASHEM, nor does it care to.
The reason that I have such conflicting desires, interests, goals and passions, is because these two parts of me, these complete opposites, are each battling for primacy over me. – And so I deeply crave being close to HASHEM -- and I also just don’t care -- Same me, just different elements coming to the surface.

Tayvah to be close to HASHEM

The Misilos Yesharim explains that by nature, I would be pulled towards HASHEM like a piece of iron is to a magnet. It is the layers and layers of physicality that block that pull. He writes that my sole focus in life should be to break through those layers of physicality until I am again drawn after HASHEM like iron to a magnet.

The role of Mitzvahs

The reason that we find serving HASHEM difficult, and the reason we find experiencing HASHEM’s presence elusive – is because of the layers and layers of desires, instincts, passions that cover up and block our Nishoma’s receptivity - almost like taking a fine radio receiver and putting it in a concrete bomb shelter. The radio waves are in the air, but blocked by the tons of concrete.
The way that one increases his spiritual receptivity is by applying the Torah – the ultimate system of self perfection. All of the mitzvahs strengthen my Nishoma and help to break the hold my Nefesh Ha’Bahami has over me. The more a person is involved in learning Torah and doing mitzvahs, the stronger the Nishoma becomes, and the easier it is for him to experience HASHEM.

The Klal Yisroel had a Tayvah to be close to HASHEM

The answer to the question seems to be that the Jewish People, while standing at the base of Har Sinai, reached unprecedented levels of spirituality. Their outside casing had very little influence on them. Their pure Nishoma came to the fore. In that state man is irresistibly drawn to his Creator - pulled, mesmerized. Consequences don’t matter; results are irrelevant; the human needs to be close to HASHEM.
HASHEM was warning Moshe of the inherent danger. Since they were going to see HASHEM so clearly, they would also feel an almost unstoppable urge for closeness to HASHEM. That pull could well be so strong that some individuals might go beyond the limit. Even though they were aware of the risk to their lives, it wouldn’t stop them because of their insatiable desire to be close to HASHEM. And so, HASHEM told Moshe, “Warn them again.”

In our world – learning to experience HASHEM

There is a very real lesson for us in this. From time to time we may find it difficult to daven properly, or it may be hard to feel the spirituality of Shabbos. These difficulties are the natural consequence of our physical side. If we aren’t constantly learning and growing in Ruchnius, the physical part of us will become stronger, making it ever more difficult to experience HASHEM’s presence.
However, we are also pure Sechel. There is a part of me that deeply craves speaking to my Maker, there is a component in me that hungers for ruchnius growth. By learning, by doing mitzvahs with fervor and energy – I bring that part to the fore, until it becomes stronger, and the physical weaker, and I grow. But not only do I grow; I grow in my desire to be more spiritual, to learn more, to daven with more kavanah. It begins an upward spiral. By putting this process into practice, a person can continue to grow throughout life, reaching higher levels and thereby fulfilling the reason he was put on this planet - to perfect himself and to enjoy that level of perfection for eternity.

For more on this topic please listen to Shmuz #74 - D’Vaikus in Our Times