Find duplicates in single array

findDupesInSingleArray.js

//Array
var y = ["?","X","Z","Q","W","W"]; 

/*OPTION ONE*/
var find_dupe = function (array) {
    /* Map Arrays, make uppercase  if wanted 
    var yy = array.map(function(x){ return x.toUpperCase() })*/
    // Sort function
    var sorted_arr = array.slice().sort(); // We use slice to clone the array so the original array won't be modified)
    var results = [];
    for (var i = 0; i < array.length -1; i++) {
        if (sorted_arr[i + 1] == sorted_arr[i]) {
            results.push(sorted_arr[i]);
        }
    }
    return results
}

//Print Duplicates: in this case [W] *If more than 2 will list dupes multiple times
console.log(find_dupe(y));