public JsonResult CreatePatient(string patient)
    {
        var jsonPatient = JsonConvert.DeserializeObject<Patient>(patient);

        if (db.Patients.Contains(jsonPatient))
        {
            db.Patients.Remove(jsonPatient);
        }
        db.Patients.Add(jsonPatient);

        return new JsonResult();
    }

public class Patient
{

    [Key]
    public string Cpr { get; set; }         //ID
    private string _firstname;
    private string _lastname;

    //public List<TestReceving> TestHandelings { get; set; }

    public string Firstname
    {
        get { return _firstname; }
        set { _firstname = value; }
    }

    public string Lastname
    {
        get { return _lastname; }
        set { _lastname = value; }
    }
    public override bool Equals(object obj)
    {
        return obj is Patient ? Cpr == (obj as Patient).Cpr : false;
    }
}

function savePatient() {
    var Patient = {
        Cpr: $("#cpr").val(),
        Lastname: $("#lastname").val(),
        Firstname: $("#firstname").val()
    };

    var dataToPost = JSON.stringify(Patient);

    $.ajax({
        type: "POST",
        url: "/Patient/CreatePatient",
        contentType: "application/json; charset=utf-8",
        data: dataToPost,
        dataType: "json",
        success: function () {
            // do what you want on success.
            alert("saved");
        },
        error: function () {
            alert("error");
        }
    });
}