gtk+-3.18.1/examples/application10 question

[Stefan Salewski <mail ssalewski de>]

gtk+-3.18.1/examples/application10

cd application10
glib-compile-schemas .
glib-compile-resources exampleapp.gresource.xml --target=resources.c --generate-source
gcc -o main main.c resources.c exampleapp.c exampleappwin.c exampleappprefs.c `pkg-config --libs --cflags 
gtk+-3.0`

I am still trying to fully understand the application10 code.
I inserted a printf call as below.

static void
example_app_prefs_dispose (GObject *object)
{
  ExampleAppPrefsPrivate *priv;
  printf("called example_app_prefs_dispose()\n");

When I open the preferences dialog window and close it again, above func
tion is called twice. 

./main exampleapp.c
called example_app_prefs_dispose()
called example_app_prefs_dispose()

Is that really desired, and why does it happen?