A short-hand solution to this problem is to create a private field to store the ID, and while creating the new Window pass the parameter to the window before showing it.
public class MyWindow : Window {
private int ID { get; set; }
public MyWindow() {
InitializeComponent();
}
public MyWindow (int id) {
ID = id;
InitializeComponent();
}
}
Now, each instance would hold different ID. You can then use this to differentiate between instance of the Window.