Don't allow redefinition of variables

[kevinbarabash]

The following test cases should be checked and any errors should be fixed:

// variables introduced with regular let
let foo = 5;
let foo = "hello";

// variables introduced by destructuring, including via the rest operator
let {x, y} = {x: 5, y: 10};
let {x, y} = {x: "hello", y: "world"};