function duplicateCount(text){
  //...
  const lowercase = text.toLowerCase()
  const arr = lowercase.split('')
  arr.sort()
  let answer = 0
  for (i=0; i<arr.length; i++) {
    if (arr[i]===arr[i+1]){
      const letter =arr[i] 
      while (arr[i+1] === letter) {
       i++
     } answer ++
    }
  }
  return answer
}