问Entity Framework 6.x Code First主键约束错误保存多对多字段

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<RecipeSet xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">  
    <Recipes>
        <Recipe author="Mr. Cook" createdDate="08/30/2018" description="Cheesesteak with mushrooms" name="Mushroom Cheesesteak" recipe_id="13">
            <Facet Taxonomy_id="1" name="Lunch" />
            <Facet Taxonomy_id="1" name="Dinner" />
            <Facet Taxonomy_id="2" name="American" />
        </Recipe>
        <Recipe author="Jane Doe" createdDate="10/01/2018" description="Vegan Hotdog" name="Vegan Hotdog" recipe_id="9">
            <Facet Taxonomy_id="1" name="Snack" />
            <Facet Taxonomy_id="1" name="Lunch" />  <!-- This is breaking EF! -->
            <Facet Taxonomy_id="2" name="Vegetarian" />
        </Recipe>
    </Recipes>
</RecipeSet>

public class Recipe
{
    [Key]
    public int Recipe_id { get; set; }  
    public string Author { get; set; }
    public DateTime CreatedDate { get; set; }
    public string Description { get; set; }
    public string Name { get; set; }

    public virtual ICollection<Facet> Facets { get; set; }
}

//Facet needs to have a unique taxonomy id and name combination
public class Facet
{
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public int Facet_id { get; set; }

    [Key]
    [Column(Order = 0)]
    public int Taxonomy_id { get; set; }

    [Key]
    [Column(Order = 1)]
    [StringLength(500)]
    public string Name { get; set; }

    public virtual ICollection<Recipe> Recipes { get; set; }
}

public class RecipeContext : DbContext
{
    public RecipeContext() : base("name=RecipeContextConn")
    {
        Database.SetInitializer<RecipeContext>(new CreateDatabaseIfNotExists<RecipeContext>());
    }

    public DbSet<Recipe> Recipe { get; set; }
    public DbSet<Facet> Facet { get; set; }

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        base.OnModelCreating(modelBuilder);
    }
}

// Automapper configuration
var config = new MapperConfiguration(cfg =>
{
    cfg.CreateMap<RecipeElement, Recipe>()
        .ForMember(dest => dest.Facets, opt => opt.MapFrom(src => src.Facet));

    cfg.CreateMap<FacetElement, Facet>();
});

Mapper = config.CreateMapper();

foreach (var recipeElement in fullRecipeSet.Recipes.Recipe)
{
    // Using Automapper to map from one object to another
    Recipe recipeDto = Mapper.Map<Recipe>(recipeElement);

    ctx.Recipe.Add(recipeDto);
}

ctx.SaveChanges();