Override theme (not-pluggable) function from child theme

Hi everybody, I have in my parent functions.php

add_action( 'after_setup_theme', 'theme_setup' );
function theme_setup() {
// Add Page Meta Fields
	require get_template_directory() . '/inc/page_meta.php';

	// Add Shortcodes
	require get_template_directory() . '/inc/shortcode.lib.php';
}

I want to override theme_setup() with this

function my_theme_setup() {
// Add Page Meta Fields
	require get_template_directory() . '/inc/page_meta.php';

	// Add CUSTOM Shortcodes
	require get_stylesheet_directory() . '/inc/shortcode.lib.php';
}

from my child theme.
I tried the methods given here but I always get an error telling me:
Fatal error: Cannot redeclare cd_meta_box_add() …
which is in page_meta.php.
Many thanks