const removeExactlyOneDuplicate = (arr) => {
  let map = new Map();

  for (let i = 0; i < arr.length; i++) {
    if (!map.has(arr[i])) {
      map.set(arr[i], 1);
    } else {
      return arr[i];
    }
  }
};

// console.log(removeExactlyOneDuplicate([1, 3, 4, 2, 2]));
console.log(removeExactlyOneDuplicate([3, 1, 3, 4, 2]));