The Meshech Chochma explains that the argument of the snake was far more subtle than simply telling Chavah to ignore G-d’s command to abstain from eating the eitz hada'as. The snake acknowledged that G-d had indeed commanded not to eat the fruit, but explained to Chavah that the reason for the prohibition is because eating the fruit would bring one to tremendous closeness with G-d -- this is the “knowledge” that would be gleaned by eating. Why then did G-d say that one who eats would die? Death, explained the snake, was not a punishment, but a consequence of the knowledge attained. It is impossible to remain involved in the temporal needs and occupations of this world and at the same time achieve the degree of spiritual ecstasy promised by eating the forbidden fruit. Therefore, one who eats must be prepared to die -- the path to spirituality demands complete surrender of existance in this world. Eating would be an aveira lishma!
The pasuk, "Lo mos temusun...," should be read as follows: "Lo!" -- No, the prohibition and punishment are not as you understand. "Mos temusun" -- Indeed, you shall certainly die if you eat from the tree, but not as a punishment, but rather, "Ki b’yom achalchem mimenu v’nifkechu eineichem" -- Because by eating your eyes shall be opened to spiritual wonder and you will no longer belong chained to mere physical existance.
After Adam and Chavah ate, they were forced to hide from G-d’s presence. Consuming the fruit produced the opposite effect they had hoped for -- they found themselves distanced from G-d, less spiritual beings than they had been earlier. G-d challenged them, “Have you eaten from the tree which I commanded not to eat from?” The pasuk repeats the command "lo tochal mimenu" to reinforce that Hashem expected obedience to the simple meaning of His words -- "Don't eat." All the philosophical justifications and elaborate explanations for why circumventing that command might be a good idea just confused and obfuscated what should have been simple.