I get the following errors when I try to insert a movie with a list of unique genres.

System.InvalidOperationException: The instance of entity type ‘Genre’ cannot be tracked because another instance with the same key value for {‘Id’} is already being tracked. When attaching existing entities, ensure that only one entity instance with a given key value is attached. Consider using ‘DbContextOptionsBuilder.EnableSensitiveDataLogging’ to see the conflicting key values.
at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.IdentityMap1.ThrowIdentityConflict(InternalEntityEntry entry) at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.IdentityMap1.Add(TKey key, InternalEntityEntry entry, Boolean updateDuplicate)
at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.IdentityMap1.Add(TKey key, InternalEntityEntry entry) at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.IdentityMap1.Add(InternalEntityEntry entry)

Question

What is the correct way to insert it?

Here are my entities

public class Movie
{
    public int Id { get; set; }
    public string Title { get; set; } = null!;
    public ICollection<Genre> Genres { get; set; } = [];
}

[Index(nameof(Name), IsUnique = true)]
public class Genre
{
    public int Id { get; set; }
    public string Name { get; set; } = null!;
    public ICollection<Movie> Movies { get; set; } = [];
}

public class AppDbContext(DbContextOptions<AppDbContext> o) : DbContext(o)
{
    public DbSet<Movie> Movies { get; set; }
    public DbSet<Genre> Genres { get; set; }
}

And my web api:

var builder = WebApplication.CreateBuilder(args);
builder.Services.AddDbContext<AppDbContext>(o => o.UseSqlite("DataSource=Movie.db"));

var app = builder.Build();
if (app.Environment.IsDevelopment())
{
    using IServiceScope scope = DevelopmentSeeder(app);
}

app.MapPost("movies", async Task<Ok> (AppDbContext db) =>
{
    Genre[] genres =
    [
        new Genre{Name="comedy"},
        new Genre{Name="action"},
        new Genre{Name="drama"},
        new Genre{Name="horror"},
    ];

    foreach (var genre in genres)
    {
        var existing = await db.Genres.FirstOrDefaultAsync(g => g.Name == genre.Name);

        if (existing is null)
        {
            db.Genres.Add(genre);
        }
        else
        {
            genre.Id = existing.Id;
        }
    }

    var movie = new Movie
    {
        Title = "I don't know",
        Genres = genres
    };

    db.Movies.Add(movie);

    await db.SaveChangesAsync();
    return TypedResults.Ok();
});

app.Run();

static IServiceScope DevelopmentSeeder(WebApplication app)
{
    var scope = app.Services.CreateScope();
    var db = scope.ServiceProvider.GetRequiredService<AppDbContext>();
    db.Database.EnsureDeleted();
    db.Database.EnsureCreated();


    Genre[] genres =
    [
        new Genre{Name="comedy"},
        new Genre{Name="action"},
    ];

    db.Genres.AddRange(genres);

    Movie movie = new Movie
    {
        Title = "Kungfu Hustle",
        Genres = genres
    };
    db.Movies.Add(movie);

    db.SaveChanges();

    return scope;
}