System.InvalidOperationException: „An entity object cannot be referenced by multiple instances of IEntityChangeTracker.”

Question

Cześć mam pytanie chciałbym napisać mechanizm płacenia w sklepie ale gdy klikam zapłać 

wyskakuje taki błąd System.InvalidOperationException: „An entity object cannot be referenced by multiple instances of IEntityChangeTracker.” błąd wyskakuje podczas db.SaveCHanges();

wiecie może jak to naprawić ?

Z góry dziękuje za pomoc :)  

using Sklep.Models;
using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Linq;
using System.Web;

namespace Sklep.Infrastructure
{
    public class OrderManager
    {
        private ISessionManager session;
        private ApplicationDbContext db;

        public OrderManager(ISessionManager session, ApplicationDbContext db)
        {
            this.session = session;
            this.db = db;
            
        }

        public const string sessionKey = "BasketKey";


        public List<OrderItems> GetBasket()
        {
            List<OrderItems> basket;

            if(session.Get<List<OrderItems>>(sessionKey)==null)
            {
                basket = new List<OrderItems>();
            }
            else
            {
                basket = session.Get<List<OrderItems>>(sessionKey) as List<OrderItems>;
            }
            return basket;
        }

        public void AddToBasket(int productId)
        {
            var basket = GetBasket();
            var basketItem = basket.Find(x => x.ProductId == productId);

            if(basketItem != null)
            {
                basketItem.Quantity++;
            }
            else
            {
                var product = db.Products.Find(productId);
                

                if(product!= null)
                {
                    var newBasketItems = new OrderItems
                    {
                        Product = product,
                        ProductId = productId,
                        Quantity = 1,
                        Price = product.Price
                    };

                    basket.Add(newBasketItems);
                }
            }

            session.Set<List<OrderItems>>(sessionKey, basket);
        }

        public int DeleteFromBasket(int productId)
        {
            var basket = GetBasket();
            
            var basketItem = basket.Find(x => x.ProductId == productId);

            if(basketItem != null)
            {
                basketItem.Quantity--;
                return basketItem.Quantity;
            }
            else
            {
                basket.Remove(basketItem);
            }

            return 0;

        }


        public decimal GetTotalValue()
        {
            var basket = GetBasket();
            return basket.Sum(x => (x.Product.Price * x.Quantity));
        }

        public int GetAmountBasketItems()
        {
            var basket = GetBasket();
            return basket.Sum(x => x.Quantity);
        }


        public Order CreateOrder(Order newOrder, string UserId)
        {
            var basket = GetBasket();
            var user = db.Users.SingleOrDefault(x => x.Id == UserId);
            newOrder.DateOfAddition = DateTime.UtcNow;
            newOrder.UserId = UserId;

            db.Orders.Add(newOrder);


            if (newOrder.OrderItems == null)
                newOrder.OrderItems = new List<OrderItems>();

            decimal basketValue = 0;
            foreach (var item in basket)
            {
                var newOrderItems = new OrderItems()
                {
                    ProductId = item.ProductId,
                    Product = item.Product,
                    Quantity = item.Quantity,
                    Price = item.Price,
                    
                };
                basketValue += (item.Price * item.Quantity);
                newOrder.OrderItems.Add(newOrderItems);
            }

            newOrder.TotalValue = basketValue;

            db.SaveChanges();

            return newOrder;
        }

        public void EmptyBasket()
        {
            
            session.Abandom();
        }
    }

}

using Microsoft.AspNet.Identity;
using Microsoft.AspNet.Identity.Owin;
using Sklep.Infrastructure;
using Sklep.Models;
using Sklep.ViewModels;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Mvc;

namespace Sklep.Controllers
{
    public class OrderController : Controller
    {
        private OrderManager orderManager;
        private ISessionManager session { get; set; }
        private ApplicationDbContext db;
        private ApplicationSignInManager _signInManager;
        private ApplicationUserManager _userManager;




        public OrderController()
        {
            session = new SessionManager();
            db = new ApplicationDbContext();
            orderManager = new OrderManager(session,db);
        }

        public OrderController(ApplicationUserManager userManager, ApplicationSignInManager signInManager)
        {
            UserManager = userManager;
            SignInManager = signInManager;
        }



        public ApplicationSignInManager SignInManager
        {
            get
            {
                return _signInManager ?? HttpContext.GetOwinContext().Get<ApplicationSignInManager>();
            }
            private set
            {
                _signInManager = value;
            }
        }

        public ApplicationUserManager UserManager
        {
            get
            {
                return _userManager ?? HttpContext.GetOwinContext().GetUserManager<ApplicationUserManager>();
            }
            private set
            {
                _userManager = value;
            }
        }


        public ActionResult Index()
        {
            var orderItems = orderManager.GetBasket();
            var totalValue = orderManager.GetTotalValue();
            OrderViewModel vm = new OrderViewModel()
            {
                OrderItems = orderItems,
                TotalValue = totalValue
            };
            ViewBag.Amount = orderItems.Count;
            return View(vm);
            
        }

        public ActionResult AddToBasket(int id)
        {
            orderManager.AddToBasket(id);
            TempData["message"] = "Pomyślnie dodano do koszyka";
            return RedirectToAction("Index", "Home");
        }

        public int AmountItems()
        {
            var basketAmount = orderManager.GetAmountBasketItems();
            return basketAmount;
        }

        public ActionResult DeleteFromBasket(int id)
        {
            var orderItems = orderManager.GetBasket();

            var basketItem = orderItems.Find(x => x.ProductId == id);

            if (basketItem.Quantity >= 2)
            {
                basketItem.Quantity--;
            }
            else
            {
                orderItems.Remove(basketItem);
            }

            var totalValue = orderManager.GetTotalValue();
            OrderViewModel vm = new OrderViewModel()
            {
                OrderItems = orderItems,
                TotalValue = totalValue
            };
            ViewBag.Amount = orderItems.Count;
            return RedirectToAction("Index", vm);

        }

        [HttpGet]
        public ActionResult Pay()
        {
            if(Request.IsAuthenticated)
            {
                var userId = User.Identity.GetUserId();
                var user = UserManager.FindById(userId);

                var order = new Order()
                {
                    FirstName = user.FirstName,
                    SecondName = user.SecondName,
                    Adress = user.Adress,
                    City = user.City,
                    ZipCode = user.ZipCode,
                    Phone = user.PhoneNumber,
                    Email = user.Email
                    
                };

                return View(order);
            }
            else
            {
                return RedirectToAction("Login", "Account", new { returnUrl = Url.Action("Pay", "Order") });
            }
        }

        [HttpPost]
        public ActionResult Pay(Order order)
        {
            if(ModelState.IsValid)
            {
                var userId = User.Identity.GetUserId();
                var newOrder = orderManager.CreateOrder(order, userId);


                var user = UserManager.FindById(userId);
                TryUpdateModel(user);
                UserManager.Update(user);

                orderManager.EmptyBasket();

                TempData["message"] = "Dziękujemy za złożenie zamówienia";
                return RedirectToAction("Index", "Home");


            }
            else
            {
                return View(order);
            }
        }
    }
}