Оформил: DeeCo
Автор: http://www.swissdelphicenter.ch

procedure TForm1.Button1Click(Sender: TObject);
 var
   sl: TStringList;
 begin
   sl := TStringList.Create;
   try
     with sl do
     begin
       // Duplicates does nothing 
      // if the list is not sorted. 
      Sorted := True;
       // Ignore attempts to add 
      // duplicate strings to the list. 
      Duplicates := dupIgnore;
       Add(Edit1.Text);
     end;
     Listbox1.Items.Assign(sl);
   finally
     sl.Free;
   end;
 end;