Let keyword doubt

Tell us what’s happening:
I have just known that if you override a variable with let keyword, it throws.
But why it is happening in case 1 only, why not happening in case 2 too.

Your code so far

//case 1
let x = 4;
x= 6;

//case 2
let y = 5;
let y = 7;

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Challenge: Explore Differences Between the var and let Keywords

Link to the challenge:

In case one you are reassigning the x variable. In case two you are re-declaring the y variable- that is not allow with the let keyword