在DbContext中保留多个具有相同键的对象

我正在尝试创建ASP。NET MVC web应用程序首先使用实体框架代码。我有两个简单的模型

 public class Assignment
    {
        public int Id { get; set; }
        [ForeignKey("Initiator")]
        public int InititatorId { get; set; }
        public virtual User Initiator { get; set; }
        public virtual List<User> Debtors { get; set; }
        public virtual List<User> Responsibles { get; set; }
        public Assignment()
        {
            Debtors = new List<User>();
            Responsibles = new List<User>();
        }
    }
 public class User
    {
        public int Id { get; set; }
        public string Name { get; set; }
        public string City { get; set; }
    }

和我的DbContext看起来像

 public class AssignmentContext : DbContext
    {
        public DbSet<Assignment> Assignments { get; set; }
        public DbSet<User> Users { get; set; }
        protected override void OnModelCreating(DbModelBuilder modelBuilder)
        {
            modelBuilder.Entity<Assignment>()
                .HasRequired(r => r.Initiator)
                .WithMany()
                .WillCascadeOnDelete(false); 
            modelBuilder.Entity<Assignment>()
                .HasMany(m => m.Responsibles)
                .WithMany()
                .Map(m =>
                {
                    m.MapLeftKey("AssignmentId");
                    m.MapRightKey("UserId(Responsible)");
                    m.ToTable("AssignmentResponsibles");
                });
            modelBuilder.Entity<Assignment>()
                .HasMany(m => m.Debtors)
                .WithMany()
                .Map(m =>
                    {
                        m.MapLeftKey("AssignmentId");
                        m.MapRightKey("UserId(Debtor)");
                        m.ToTable("AssignmentDebtors");
                    });
        }

是否有一种方法可以在债务人和责任人列表中保持相同的用户实例?

因为当我试图保存更改时，我得到了错误。

An object with the same key already exists in the ObjectStateManager. The ObjectStateManager cannot track multiple objects with the same key.

我需要这样做，因为这是应用程序逻辑所要求的。

提前感谢。

更新:

在HttpPost上出现错误(在控制台应用程序示例中出现相同的错误):

 [HttpPost]
        [ValidateAntiForgeryToken]
        public ActionResult Edit([Bind(Include = "Id,Text,CreationDate,InititatorId,Debtors,Responsibles")] Assignment assignment)
        {
            if (ModelState.IsValid)
            {
                assignment.Debtors.RemoveAll(r => r.Name == "Roman");
                db.Entry(assignment).State = EntityState.Modified; // HERE!
                db.SaveChanges();
                return RedirectToAction("Index");
            }
            ViewBag.InititatorId = new SelectList(db.Users, "Id", "Name", assignment.InititatorId);
            return View(assignment);
        }

视图中的负债者和责任者元素通过隐藏输入传递。