function duplicateCount(text){
  var lower = text.toLowerCase();
  var count = 0;
  var used = [];
  
  lower.split('').forEach(
      function(letter) {if (!used.includes(letter) && (lower.split(letter).length - 1) > 1) {count++; used.push(letter);}});
     //
    console.log(used);
    console.log(count);
  return count;
}

// 1 1 1 1 1 
duplicateCount("Hollla soy una prueba");

//      Sample forEach()
// before
for (let i = 0; i < items.length; i++) {
  copyItems.push(items[i]);
}

// after
items.forEach((item) => {
  copyItems.push(item);
});