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// show elements with duplicated id's
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]++;
}

for(var key in ids) {
    var value = ids[key];
    if (value > 1) {
        console.log(value, key);
    }
}