JSFiddle - React, Tailwind, and code Playground

JavaScript

function hasDupes(arr) {
  /* temporary object */
  var uniqOb = {};
  /* create object attribute with name=value in array, this will not keep dupes*/
  for (var i in arr)
    uniqOb[arr[i]] = "";
  /* if object's attributes match array, then no dupes! */
  if (arr.length == Object.keys(uniqOb).length)
    alert('NO dupes');
  else
    alert('HAS dupes');


}
var arr = ["1/1/2016", "1/1/2016", "2/1/2016"];
hasDupes(arr);