MVC 5中带有EF 6的错误“ System.InvalidOperationException：IEntityChangeTracker的多个实例无法引用一个实体对象。”

这是我将产品添加到数据库中的代码：

public void AddToCart(Product item)
{
    // Get the matching cart and album instances
    var cartItem = entities.Orders.FirstOrDefault(
        c => c.OrderGUID == ShoppingCartId
        && c.OrderItems.Where(p=>p.ProductId == item.ProductId).FirstOrDefault().ProductId == item.ProductId);

    if (cartItem == null)
    {
        // Create a new cart item if no cart item exists
        cartItem = new Order
        {
            InvoiceNumber = Guid.NewGuid().ToString(),
            OrderDate=DateTime.Now,
            OrderGUID = ShoppingCartId
        };
        entities.Orders.Add(cartItem);
    }
    else
    {
        // If the item does exist in the cart, 
        // then add one to the quantity
        cartItem.OrderItems.Where(p => p.ProductId == item.ProductId).SingleOrDefault().ProductQuantity++;
    }
    // Save changes
    entities.SaveChanges();

    OrderItem oi = new OrderItem()
    {
        OrderGUID = ShoppingCartId,
        ProductId = item.ProductId,
        ProductQuantity = item.Quantity,
        ProductPrice = item.ProductPrice,
        Product = item
    };

    entities.OrderItems.Add(oi);
    entities.SaveChanges();
}

当它碰到这条线

entities.OrderItems.Add(oi);

如果当我得到上述错误。有任何想法吗？