This is an efficient approach to finding duplicates in an array or eliminating duplicates from an array.

findeDupes.js

var a = ["a","a","b","c","c"];
var dupes = a.filter((value,index,self)=>{ return (self.indexOf(value) !== index )});
var cleanArray = a.filter((value,index,self)=>{ return (self.indexOf(value) === index )});
console.log(dupes);
console.log(cleanArray);