const arr = [3, 5, 5, 4, 3, 4, 8, 8, 2]

function findSingle(arr) {
  const set = new Set()
  while(arr.length) {
    let temp = arr.pop()
    if(!set.has(temp)) set.add(temp)
    else set.delete(temp)
  }
  return Array.from(set.values()).pop() || null
}

findSingle(arr)