docs: def duplicate(self):
          '''
          Returns a copy of the current group, including its lines.
          @returns: Group
          '''
          instance = self.__class__(identifier=self.identifier,
                                    name=self.name, description=self.description,
                                    currency=self.currency, status=self.status,
                                    date=self.date, due_date=self.due_date,
                                    terms=self.terms,
                                    seller=self.seller.duplicate(),
                                    buyer=self.buyer.duplicate(),
                                    shipping=self.shipping.duplicate(),
                                    mentions=self.mentions,)
          for group in self.groups:
              instance.groups.append(group.duplicate())
          for method in self.deliveries:
              instance.deliveries.append(method.duplicate())
          for payment in self.payments:
              instance.payments.append(payment.duplicate())
  
          return instance