System.InvalidOperationException: “The instance of entity type ‘JoinEntity’ cannot be tracked because another instance with the key value ‘{TestEntity1Id: 11, TestEntity2Id: 59}’ is already being tracked. When attaching existing entities, ensure that only one entity instance with a given key value is attached.”

I`ve recreated the error that occured in my project. For testing sakes I use InMemory provider. I have a simple N:N relationship with custom join entity and navigations to and from join entity:

namespace ConsoleApp2
{
    public class TestDbContext : DbContext
    {
        public DbSet<TestEntity1> TestEntity1 { get; set; }
        public DbSet<TestEntity2> TestEntity2 { get; set; }
        public DbSet<JoinEntity> JoinEntity { get; set; }
        protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
        {
            optionsBuilder.UseInMemoryDatabase("TestDB");
            optionsBuilder.EnableSensitiveDataLogging();
            optionsBuilder.LogTo(Console.WriteLine);
            optionsBuilder.EnableDetailedErrors();
        }
        protected override void OnModelCreating(ModelBuilder builder)
        {
     
            builder.Entity<TestEntity1>()
                .HasKey(x => x.Id);
            builder.Entity<TestEntity2>()
                .HasKey(x => x.Id);
            builder.Entity<TestEntity1>()
                .HasMany(e => e.TestEntity2List)
                .WithMany(e => e.TestEntity1List)
                .UsingEntity<JoinEntity>();
        }
    }
    public class TestEntity1
    {
        public int Id { get; set; }
        public List<TestEntity2> TestEntity2List { get; set; } = new();
        public List<JoinEntity> JoinEntityList { get; set; } = new();
    }
    public class TestEntity2 
    {
        public int Id { get; set; }
        public List<TestEntity1> TestEntity1List { get; set; } = new();
        public List<JoinEntity> JoinEntityList { get; set; } = new();
    }
    public class JoinEntity
    {
        public TestEntity1 TestEntity1 { get; set; }
        public TestEntity2 TestEntity2 { get; set; }
        public int TestEntity1Id { get; set; }
        public int TestEntity2Id { get;set; }
        public int Value { get; set; }
    }
}

For each instance of TestEntity1 3 instances of TestEntity2 are added into navigation collection. And for each relation, new instance of JoinEntity is created:

TestDbContext dbContext = new TestDbContext();


var entity1List = Enumerable.Range(1, 50)
    .Select(x => new TestEntity1 { Id = x })
    .ToList();

var entity2List = Enumerable.Range(1, 50)
    .Select(x => new TestEntity2 { Id = x + 52 })
    .ToList();

int numberOfBoundEntities = 3;
Random random = new Random();

foreach (var entity1 in entity1List)
{
    for (int i = 0; i < numberOfBoundEntities; i++)
    {
        TestEntity2 entity2 = entity2List[random.Next(0, entity2List.Count)];
        JoinEntity joinEntity = new() 
        { 
            Value = random.Next(),
            TestEntity1 = entity1,
            TestEntity2 = entity2,
            TestEntity1Id = entity1.Id,
            TestEntity2Id = entity2.Id,
        };

        entity1.TestEntity2List.Add(entity2);
        entity1.JoinEntityList.Add(joinEntity);

        entity2.TestEntity1List.Add(entity1);
        entity2.JoinEntityList.Add(joinEntity);
    }
}
dbContext.TestEntity1.AddRange(entity1List); **Here occurs the error**
dbContext.TestEntity2.AddRange(entity2List);
dbContext.SaveChanges();
Console.WriteLine(0);

I`ve tryed not to explicitly add instances of JoinEntity to collections of entity1 and entity2, but in this case EF generates new instances of JoinEntity (obvious)

All properties of JoinEntity are configured on convention so that’s definitely not the problem.