JSFiddle - React, Tailwind, and code Playground

JavaScript

var arr = [{
    id: 1,
    username: 'fred'
}, {
    id: 2,
    username: 'bill'
}, {
    id: 3,
    username: 'ted'
}];

function add(obj) {    
  if (arr.map(item=> item !== obj)){
        	arr.push(obj);
        }
}

add({
    id: 3,
    username: 'ted'
});
console.log(arr);