Insert/update multiple objects

I have the following class:

public class Location
 {
     public int Id { get; set; }
     public string Description { get; set; }
     public string CarId { get; set; } 
     public Car Car { get; set; }
 }
 
 public class Car
 {
     public string Id { get; set; }
     public string Color { get; set; } 
 }
 

And a view:

    <div>@Html.LabelFor(location => location.Description)</div>
     <div>@Html.EditorFor(location => location.Description)</div>
 
     <div>@Html.LabelFor(location => location.Car.Id)</div>
     <div>@Html.EditorFor(location => location.Car.Id)</div>
 
     <div>@Html.LabelFor(location => location.Car.Color)</div>
     <div>@Html.EditorFor(location => location.Car.Color)</div>
 

When i try this:

    [HttpPost]
     public ActionResult Create(Location location)
     {
         if (ModelState.IsValid) 
         {
             Car car = db.Car.Find(location.Car.Id);
 
             if (car != null)
                 db.Entry(car).CurrentValues.SetValues(location.Car);
             else
                 db.Car.Add(location.Car);
 
             db.Location.Add(locacao);
             db.SaveChanges();  
 
             return RedirectToAction("Index");
         }
 
         return View(locacao);
     }
 

it breaks in 'db.SaveChanges' cause it says 'Cannot insert duplicate key in object 'dbo.Car'.'

If I remove 'db.Location.Add(locacao);', so it works pretty fine for the car(insert and update), but no location is added in database.

How can I do to insert a new car when there is not car's id in database, update when there is, and insert a new location?

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