System.InvalidOperationException: The instance of entity type 'RigheOrdine' cannot be tracked because another instance with the same key value for {'Id', 'CodartVar'} is already being tracked. When attaching existing entities, ensure that only one entity instance with a given key value is attached. Consider using 'DbContextOptionsBuilder.EnableSensitiveDataLogging' to see the conflicting key values.

public async Task<int> AddOrderAsync(CreateOrderModel model, User user)
        {
            int affectedRows = 0;
            var level = IsolationLevel.Serializable;
            using (var transaction = _ctx.Database.BeginTransaction(level))
            {
                try
                {
                    int lastId = await GetLastId();
                    Orders Orders = CreteOrders(model, lastId);
                    user.Orders.Add(Orders);

                    using (var crypto = new RNGCryptoServiceProvider())
                    {
                        int retries = 5;
                        do
                        {
                            var pk = crypto.CreateNumberBetween(100000000000000, 1000000000000000);
                            Orders.CodEst = pk;

                            foreach (var item in Orders.OrderItems)
                                item.RoaCodEst = pk;

                            try
                            {
                                affectedRows = await _ctx.SaveChangesAsync();
                            }
                            catch (DbUpdateException) when (--retries >= 0)
                            {
                            }
                            catch (Exception ex)
                            {
                                string[] parametri = new string[] { model.Codcli, model.Divisione, model.Codage };
                                if (affectedRows == 0) throw new OrderNotSavedException(ex, parametri);
                            }
                        } while (affectedRows == 0);
                    }

                    transaction.Commit();
                    return affectedRows;
                }
                catch (Exception ex)
                {
                    string[] parametri = new string[] { model.Codcli, model.Divisione, model.Codage };
                    throw new OrderNotSavedException(ex, parametri);
                }
            }
        }

private static Ordcli CreteOrders(CreateOrderModel model, int lastId)
        {
            return new Order
            {
                Id = 0000000000000000,
                Anno = model.Anno,
                ...,
                OrderItems = model.OrderItems.Select((item) => new Righe
                {
                    Id = lastId,
                    CodartVar = item.Codvar == null ? item.Codart : item.Codart + "/" + item.Codvar,
                    Codart = item.Codart,
                    Codvar = item.Codvar,
                    Descri = item.Descri,
                    Quanti = item.Quanti,
                    Prezzo = item.Prezzo,
                    Implor = item.Quanti * item.Prezzo,
                    Numrig = item.Numrig,
                    CodEst = 0000000000000000
                }).ToList()
            };
        }

public async Task DeleteOrder(long id)
        {
            var ordine = await _ctx.Ordine.SingleOrDefaultAsync(x => x.CodEst == id);
            _ctx.Ordcli.Remove(ordine);
            await _ctx.SaveChangesAsync();
        }
        public async Task SoftDelete(long id)
        {
            var ordine = await _ctx.Ordine.SingleOrDefaultAsync(x => x.CodEst == id);
            ordine.Deleted = 1;
            _ctx.Ordine.Update(ordine);
            await _ctx.SaveChangesAsync();
        }