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",
};