ASP.NET MVC Check if there is a change in the model for an If statement

I am trying to put some validation logic into my controller but I'm having a little trouble.

Here's the code:

 if (holidayRequestForm.Approved == true)
            {
                TempData["msgAlreadyApproved"] = "This Holiday Has already been approved";
                return RedirectToAction("Index", "HolidayRequestFroms");
            }

My problem is I want to throw an error message for when a user attempts to approve a holiday request that has already been approved but obviously the code above will throw the message when any holiday get set to approved.

How would I go by checking if a change happens in the model and then throwing the temp message if no change has taking place.

thanks