Multiple properties with the same key in object literals should be avoided.

When an object literal has multiple properties with the same key, only the key-value pair that appears last in the source code is considered. This renders all the previous property initializations with the same key useless.

Bad Practice

const obj = {
    city: "nyc",
    city: "ohio"
};
console.log(obj.city); // ohio

const obj = {
    [0x1]: "nyc",
    [1]: "ohio"
};
console.log(obj[1]); // ohio

Recommended

const foo = {
    city: "nyc",
    country: "US"
};