No Dupe Keys

Disallows duplicate keys in object literals.

Setting the same key multiple times in an object literal will override other assignments to that key and can cause unexpected behaviour.

Invalid:

const foo = {
  bar: "baz",
  bar: "qux",
};

const foo = {
  "bar": "baz",
  bar: "qux",
};

const foo = {
  0x1: "baz",
  1: "qux",
};

Valid:

const foo = {
  bar: "baz",
  quxx: "qux",
};