Easiest way to find duplicate values in a JavaScript array

duplicate_elements_in_array.js

var dupEleArr = [1, 1, 1, 1, 1, 1, 1, 1, 2, 2, 2, 2, 2, 2, 1, 1, 1, 1, 2, 2, 2, 20, 3, 3, 3, 32, 324, 5, 52];

/*
	Sort the given array and filter the array by checking 
	If two adjucent elements are equal and then the next immediate
	element should not be equal...
	Note: Tested with array of numbers, Not tested with strings
*/
var dupEle = dupEleArr.sort().filter(function(ele,i,arr){
   return arr.length > 3 ? (arr[i] == arr[i+1] && arr[i+1] != arr[i+2]) : (arr.length == 2) ? (arr[i] == arr[i+1]) : true;
})

//document.querySelector("body").innerHTML = JSON.stringify(dupEle);