Напечатать буквы, входящие в текст не менее двух раз - Pascal

var s: string;
    one, two: set of char;
    c: char;
    i: integer;
begin
  one := [];
  two := [];
  writeln('Enter string:');
  readln(s);
  for i := 1 to length (s)
    do if s[i] = '.'
      then break
      else if s[i] in ['a'..'z']
        then if s[i] in one
          then include(two, s[i])
          else include(one, s[i]);
  write('Small Latin letters which not less than two times occur in the text');
  if two = []
    then write(' not found.')
    else begin
      writeln(':');
      for c := 'a' to 'z' do if c in two then write(c);
    end;
  readln
end.