Can I redeclare let and const variables

No, you cannot redeclare let and const variables. If you do, it throws below error

Uncaught SyntaxError: Identifier 'someVariable' has already been declared

Explanation: The variable declaration with var keyword refers to a function scope and the variable is treated as if it were declared at the top of the enclosing scope due to hoisting feature. So all the multiple declarations contributing to the same hoisted variable without any error. Let's take an example of re-declaring variables in the same scope for both var and let/const variables.

var name = "John";
function myFunc() {
var name = "Nick";
var name = "Abraham"; // Re-assigned in the same function block
alert(name); // Abraham
}
myFunc();
alert(name); // John

The block-scoped multi-declaration throws syntax error,

let name = "John";
function myFunc() {
let name = "Nick";
let name = "Abraham"; // Uncaught SyntaxError: Identifier 'name' has already been declared
alert(name);
}

myFunc();
alert(name);