The Torah divides its warning into four categories. Al Yeirach Levavchem, Al Tir'u, V'Al Tachpezu, V'Al Ta'artzu MiPneihem. Rav Yerucham explains. The Torah is teaching that the battle with evil is constant and never-ending. It is present at every stage of our lives, and everywhere we turn.
Every move is an opening for evil, demanding vigilance.
In other words, each situation deserves a separate warning. There is no escaping the responsibility with which G-d has entrusted us. Man must constantly be on the lookout. He must learn to distinguish the silent demand of his conscience from the raucous laughter of a beckoning world. To remain coolly unaware of the consequence of his actions is to fall victim to Amalek, convinced by the snake that nothing really matters.