Help me shuffle

[Stremik]

I want the code below to shuffle alphabet letters that are in the array.
The code works but as it shuffles, it produces doubles and triples of some letters and I end up with an alphabet that is missing some letters while
at the same time has doubles and triples of some.
I don't understand why!
Can anyone help?
Thank you.

  int num=atoi(argv[1]);
  srand(num);
  int i, j;
  char tmp;
  char arr[26]={'a','b','c','d',....etc};

  for(i=0;i<26;i++){
      j=rand()%26;
      tmp=arr[i];
      arr[i]=arr[j];
      arr[j]=tmp;
  }