layerDuplicates.js

var layerNum = app.activeDocument.layers.length;
var arr = [];
 for (var i = 0; i < layerNum; i++) {
arr[i]  = '"'+app.activeDocument.layers[i].name+'"';

}

// taken from: http://stackoverflow.com/questions/840781/easiest-way-to-find-duplicate-values-in-a-javascript-array

var sorted_arr = arr.sort(); // You can define the comparing function here. 
                             // JS by default uses a crappy string compare.
var results = [];
for (var i = 0; i < arr.length - 1; i++) {
    if (sorted_arr[i + 1] == sorted_arr[i]) {
        results.push(sorted_arr[i]);
    }
}

prompt("Layers Names:", results);