function Repetidos(Lista: TStrings): Integer;
var
  i, cnt: integer;
  S: TStringList;
begin
  i:= 0;
  Result := 0;
  S := TStringList.Create;
  S.Assign(Lista);
  while S.Count > i do
    if S.IndexOf(S.Strings[i])<>i then
    begin
      S.Delete( i );
      inc(Result);
    end
    else
      inc( i );
  S.Free;
end;

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