Javascript - Elements added into an array not in correct place

I have a little problem to add some element in place into an array. I'm trying to add after the last lowercase letter that is z the uppercase letter of the alphabet. I'm using this code. I've noticed that the lower char z is added after all the uppercase letters are added into the array, also the first two uppercase letters are repeated before the lower z is added. How I can fix this?

onst splitMatrix = async (inputMatrix: string) => {
  let splitted: Array<string> = inputMatrix.split(''); 
  let index: number = splitted.indexOf("z");
  //let uppercaseChars: Array<string> = [];
  //
  index + 1;
  for (let i = 0; i < 23;i++){
    let char: string = splitted[i].toUpperCase();
    //uppercaseChars.push(char);
    splitted.splice(index + i, 0, char);
    console.log(char)
  }
  //console.log(uppercaseChars)
  //console.log(splitted)
  return splitted;
}

This is what I will have as result of the code

[
  'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i',
  'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't',
  'u', 'w', 'x', 'y', 'A', 'B', 'C', 'D', 'E',
  'F', 'G', 'H', 'I', 'L', 'M', 'N', 'O', 'P',
  'Q', 'R', 'S', 'T', 'U', 'W', 'X', 'Y', 'A',
  'z', '0', '1', '2', '3', '4', '5', '6', '7',
  '8', '9', '@', '-', '_', '+', '*', '(', ')',
  ',', '.', '#'
]

My idea is to add a control on the chars to avoid duplicated entries into the array, but I'm open to any tips to solve this. Thank you

Iterate the array, stop at the original length (len), and push the uppercased letters to the end:

const splitMatrix = inputMatrix => {
  const splitted = inputMatrix.split(''); 
  
  const len = splitted.length;
  
  for (let i = 0; i < len; i++){
    splitted.push(splitted[i].toUpperCase());
  }

  return splitted;
}

const result = splitMatrix('abcdefghijklmnopqrstuvwxyz');

console.log(result);

Another option is to map the lowercase characters array to an array of uppercase letters, and then concat it with the lowercase array:

const splitMatrix = inputMatrix => {
  const lowerCase = inputMatrix.split(''); 
  
  return lowerCase.concat(
    lowerCase.map(l => l.toUpperCase())
  );
}

const result = splitMatrix('abcdefghijklmnopqrstuvwxyz');

console.log(result);