var indexs = "Prashant".MultipleIndex('a');

//Extension Method's Class
    public static class Extensions
    {
         static int i = 0;

         public static int[] MultipleIndex(this string StringValue, char chChar)
         {

           var indexs = from rgChar in StringValue
                        where rgChar == chChar && i != StringValue.IndexOf(rgChar, i + 1)
                        select new { Index = StringValue.IndexOf(rgChar, i + 1), Increament = (i = i + StringValue.IndexOf(rgChar)) };
            i = 0;
            return indexs.Select(p => p.Index).ToArray<int>();
          }
    }