//***DISCLAIMER: ONLY WORKS FOR POSITIVE NUMBERS***
let arr = [4, 7, 8, 9, 3, 6, 2, 2]

arr.forEach((val, i) => {
  //Use the absolute value because someone else might have made it negative
  val = Math.abs(val)
  console.log(arr)
  //If the value in the val index is less than 0, we know that the first occurence of the val has changed it to negative. So this is the duplicate one!
  if (arr[val] < 0) {
    console.log('first duplicate of ' + val + ' at position ' + i)
    return
  }
  //If index is out of bounds, then just assign the negative of the value, else assign negative of whatever value is there in that position already
  arr[val] = !arr[val] ? val * -1 : arr[val] * -1
})