function findDupes(arr) {
	
  for (let i = 0; i < arr.length; i++){
  	if (hash[arr[i]] === "true"){
    	dupes.push(arr[i]);
    }
    
  	hash[arr[i]] = "true" ;
  }
  
  console.log(dupes);
}

let arr = [1, 5, 6, 6, 6, 5 ,6 ,7 ,3, 2, 1, 3];
let dupes = [];
let hash = {};

findDupes(arr);