function Repetidos(Lista: TStrings): Integer;
var
  i: integer;
begin
  Result := 0;
  for i  := 0 to Lista.Count-1 do
    if Lista.IndexOf(Lista.Strings[i] )<>i then
      Inc(Result);
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
  if opendialog1.execute then
    listbox1.items.loadfromfile(opendialog1.filename);
  ShowMessage(IntToStr(Repetidos(Listbox1.items)));
 //Displays
end;