JSFiddle - React, Tailwind, and code Playground

JavaScript

var objectList = [
    {qn: 0,points: "2"},
    {qn: 1,points: "2"},
    {qn: 2,points: "6"},
    {qn: 2,points: "2"},
    {qn: 3,points: "3"}
];

function find_duplicate_value(objList, field) {

    if (objList.length == 0) 
        return null;
    
    var tmp = [];
    
    for (var i = 0; i < objList.length; i++) {
        var val = objList[i][field];
        var pos = tmp.indexOf(val)

        if (pos > -1) 
            return pos;

        tmp.push(val);
    }
    return null;
}

var duplicate = find_duplicate_value(objectList, 'qn');
alert( ( (duplicate === null) ? "Nothing" : duplicate ) + ' is duplicated');