code_list = []
        for form in self.forms:
            if (
                form.cleaned_data
                and 'code' in form.cleaned_data
                and not form.cleaned_data['DELETE']
            ):
                if cleaned_data.get('id', None):  # existing item
                    accepted_items_in_db = 1
                else:  # new item
                    accepted_items_in_db = 0

                if (
                    form.cleaned_data['code'] in code_list
                    or Material.objects.filter(code=form.cleaned_data['code']).count() > accepted_items_in_db
                ):
                    form.add_error('code', _('This name is duplicated'))
                else:
                    code_list.append(form.cleaned_data['code'])