spawn objects in random locations, check if location already taken

Hi! you'll see, i have this scripts that i made, i have 8 letters i wanna spawn in 11 possible spots, i want each letter to be assign to one unique location, but when i run the script, some letters are spawned at the same spot. What am i doing wrong? :I Thnks! :b

 var letter : GameObject[]; //prefabs that will be spawned
 
 var letterSpot : GameObject[]; //Empty GameObjects where the prefabs will spawn
 var spotUsed : boolean[]; //wether the spot is already used or not
 
 private var letterIndex : int;
 
 
 function Start ()
 {
     PutLetter(0);
     PutLetter(1);
     PutLetter(2);
     PutLetter(3);
     PutLetter(4);
     PutLetter(5);
     PutLetter(6);
     PutLetter(7);
 
     
 }
 
 function PutLetter (i : int)
 {
         letterIndex = Random.Range(0,10);
     while (spotUsed == true)
     {
         letterIndex = Random.Range(0,10);
     }
     Instantiate(letter[i], letterSpot[letterIndex].transform.position, letterSpot[letterIndex].transform.rotation);
     spotUsed[letterIndex]=true;
 }