[Deff]

function indexRandom(lng){
     var a = {},i,out=[],n=0; 
     while (n!=lng){
       i = Math.floor(lng*Math.random());
       if(typeof(a[i])=='undefined'){
         a[i]=i; out.push(i);
         n++;
       }
     }
    return out;
  }
 
  var arr = [],ind = indexRandom(20),k;
  for(i=0;i<20;i++) arr[i]=ind[i]%(k=i<10?20:3);

  alert(arr)
  alert(arr.join(''))