Go gorm many2many中的重复ID

I am having an issue with saving many2many in gorm with go. The problem is that I want a book to have multiple genres, but when I try to use the Association method to add genres to the books, I get the following error

Error 1062: Duplicate entry '2debe760-2cf8-4dff-80dd-a912ff6f8176' for key 'book_id'

I do know that the SQL generated by the gorm is false which gives the error above. The question is what should I do to avoid the check for "where not exist"? That check is not needed at all for many to many relationship. I dont understand the purpose of the check.

INSERT INTO `book_genres` (`book_id`,`genre_id`) SELECT 'e038ba55-ecd9-41a3-a1ea-36da86d465ea','3df46a42-c1ce-416c-b246-753b81fc2246' FROM DUAL WHERE NOT EXISTS (SELECT * FROM `book_genres` WHERE `book_id` = 'e038ba55-ecd9-41a3-a1ea-36da86d465ea' AND `genre_id` = '3df46a42-c1ce-416c-b246-753b81fc2246')

This is my setup, what I am missing to ensure I don't get the error?

type Book struct {
        Identifiable
        Name      string        `sql:"type:varchar(255);not null" json:"name"`
        Pages     int           `sql:"type:integer;not null"`
        AuthorId  uuid.UUID     `sql:"type:varchar(36);foreign_key"`
        Genres    []Genre       `gorm:"many2many:book_genres;"`
    }

type Genre struct {
        Identifiable
        Name            string  `sql:"type:varchar(255);not null" json:"name"`
        Books []*Book
    }

var auth domain.Author;
auth.Age = 59;
auth.Name = "Fyodor Dostoyevsky";
fmt.Println(auth)

var drama domain.Genre
drama.Name = "Drama"
var thriller domain.Genre
thriller.Name = "Thriller"

var genres []domain.Genre
genres = append(genres, drama, thriller)
engine.Save(&drama)
engine.Save(&thriller)

var book1 domain.Book;
book1.Name = "Idiot";
book1.Pages = 11000;
book1.AuthorId = auth.Id;

var book2 domain.Book;
book2.Name = "Crime and Murder";
book2.Pages = 854;
book2.AuthorId = auth.Id;

engine.Save(&auth)

engine.Model(&auth).Association("Books").Append(&book2)
engine.Model(&auth).Association("Books").Append(&book1)

err := engine.Debug().Model(&book1).Association("Genres").Append([]domain.Genre{drama, thriller});
if err != nil {
    fmt.Println(err)
}