Django zapisuje model do bazydanych podwójnie

Question

Mam kod zapisujący z google books api.

            for book_data in data['items']:
                volume_info = book_data['volumeInfo']
                title = volume_info['title']
                genres = volume_info.get('categories')
                authors = volume_info.get('authors')
                description = volume_info.get('description')
                if not Books.objects.filter(title=title).exists():
                    book = Books.objects.create(title=title, description=description)

                    # Does authors exists in database?
                    existing_authors = Authors.objects.filter(author_name__in=authors)
                    existing_authors_names = {authors.author_name for authors in existing_authors}

                    # Create a list of missing authors
                    missing_authors = [
                        Authors(author_name=author_name)
                        for author_name in authors
                        if author_name not in existing_authors_names
                    ]
                    print(missing_authors)
                    # Creating author before adding it to relation
                    if missing_authors:
                        missing_authors = Authors.objects.bulk_create(missing_authors)
                        for m in missing_authors:
                            m.save()

                    # Adding authors to book relation
                    book.authors.add(*existing_authors, *missing_authors)

                    # Does genres exists in database?
                    existing_genres = Genres.objects.filter(genres__in=genres)
                    existing_genres_name = {genres.genres for genres in existing_genres}
                    # Create a list of missing genres
                    missing_genres = [
                        Genres(genres=genres)
                        for genres in genres
                        if genres not in existing_genres_name
                    ]
                    # Creating genre before adding it to relation
                    if missing_genres:
                        missing_genres = Genres.objects.bulk_create(missing_genres)
                        for m in missing_genres:
                            m.save()
                    # Adding genres to book relation
                    book.genres.add(*existing_genres, *missing_genres)

Moim problemem jest to że zapisuje on po 2 razy tego samego autora i genre( np. J.R.R. Tolkien, Fiction) ale tylko jeden ma relację z modelem Books. Jak zrobić żeby zapisywało tylko raz Autora i Genre