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procedure Tform2.fuelle_listbox1f2Kla(sender:tobject);
var kb : string;
begin
  listbox1.Visible := false; listbox1.Clear;
  kb := 'x';
  dbgrid1.DataSource.DataSet.First;
  while not dbgrid1.DataSource.DataSet.Eof do
  begin
    if dbgrid1.DataSource.DataSet.fieldbyname['Klasse'].asstring[1] <> kb then //Mecker!
    begin
      listbox1.Items.Add(dbgrid1.DataSource.DataSet.fieldbyname['Klasse'].asstring[1]);
      kb := dbgrid1.DataSource.DataSet.fieldbyname['Klasse'].asstring[1];
    end;
    dbgrid1.DataSource.DataSet.next;
  end;
  //SetLbWidth(Listbox1); SetScrollWidth(listbox1);
  //listbox1.Height := listbox1.ItemHeight * anzkla + 20;
  listbox1.Visible := true;
end;					

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var s: String;
    c: character;
//...
    s := dbgrid1.DataSource.DataSet.fieldbyname['Klasse'].asstring;
    c := s[1]
    if c <> kb then //Mecker!
//...					

 
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{ ... }
if dbgrid1.DataSource.DataSet.fieldbyname('Klasse').asstring[1] <> kb then