after_setup_theme is always running

What I want it to do:
install some default categories when I initialize my theme.

What it does:
installs the default themes all the time, even after the theme is activated. Result: if I have a default categorie “book” and I rename it to “books”, I have instantly two categories: “book” and “books”.

function my_setup() {
$catz = array("test1", "test foto\'s", "test / testing");
foreach ( $catz as $cat ) {
	wp_insert_term(
		$cat,
		'category'
	);
}
}
add_action( 'after_setup_theme', 'my_setup' );

What goes wrong, and how can I get it right to only run the my_setup function once.

[Moderator Note: No bumping, thank you.]