Question

How to check the tags that contains duplicate ids in javascript?

Answer 1

Try this:

var nodes = document.querySelectorAll('[id]');
var ids = {};
var totalNodes = nodes.length;

for(var i=0; i<totalNodes; i++) {
    var currentId = nodes[i].id ? nodes[i].id : "undefined";
    if(isNaN(ids[currentId])) {
        ids[currentId] = 0;
    }                 
    ids[currentId]++;
}

console.log(ids);

http://jsfiddle.net/sekVp/1