var unSortedArr = [2, 3, 2, 5, 4, 5, 5, 9, 6, 8, 8, 7];
var sum = 10;

function pairMatchingSum (arr, sum) {
  var start, end, tempSum;

  var i = 0 ;
  var j = arr.length - 1;

  // Create a new Map instance
  var s = new Map();
  var sortedArr = arr.sort();

  while (i !== j) {
    start = sortedArr[i];
    end = sortedArr[j];
    tempSum = start + end;

    if (tempSum === sum) {
      // Add matching pair to the Map. Map does not allow duplicate keys.
      s.set(start +'-'+ end, true);
      i++;
      j--;

    } else if (tempSum > sum) {
      j--;

    } else {
      i++;
    }
  }

  console.log("Pairs matching the input sum in the given array without duplicates = ", [...s.keys()]);
  // Pairs matching the input sum in the given array without duplicates =  ["2-8", "3-7", "4-6", "5-5"]
}

pairMatchingSum(unSortedArr, sum);