var hat = require('hat');
var _ = require('lodash');

var rack = {};

for(var i = 0; i < 1000000; i++) {
  var id = hat();
  rack[id] = rack[id] ? rack[id] + 1 : 1;
}

var dupes = _.filter(rack, function(a) { return a > 1 });
console.log(dupes);