These are my entities:

public class Order
{
    [Key]
    public long OrderId { get; set; }
    public string CustomerId { get; set; }

    [ForeignKey("CustomerID")]
    [Required]
    public Customer Customer { get; set; }
    public long EmployeeId { get; set; }

    [ForeignKey("EmployeeID")]
    public Employee Employee { get; set; }
    ...
    public ICollection<OrderDetail> OrderDetails { get; set; }
}

public class OrderDetail
{
    // public long Id { get; set; }

    [Key, Column(Order = 0)]
    public long OrderId { get; set; }

    [ForeignKey("OrderID")]
    public Order Order { get; set; }
    public long ProductId { get; set; }
    public Product Product { get; set; }
    public double UnitPrice { get; set; }
    public long Quantity { get; set; }
    public double Discount { get; set; }
}

public class Category
{
    [Key]
    public long CategoryId { get; set; }

    public string CategoryName { get; set; }
}

I need to implement AddOrderAsync and UpdateOrderAsync methods. I have tried a lot of variants but getting different types of errors like

System.InvalidOperationException : The instance of entity type ‘Category’ cannot be tracked because another instance with the same key value for {‘CategoryId’} is already being tracked.

public async Task<long> AddOrderAsync(RepositoryOrder order)
{        
    var entityOrder = order.ToEntityOrder();
    this.context.Orders.Add(entityOrder);

    // Add the order details
    foreach (var detail in order.OrderDetails)
    {
        this.context.OrderDetails.Add(detail.ToEntityOrderDetail());
    }

    // Save changes
    await this.context.SaveChangesAsync();

    // Commit transaction
    await transaction.CommitAsync();
    return entityOrder.OrderId;
}