Fatal error: Cannot redeclare bp_dtheme_ajax_querystring()

I’m designing a new child theme for Buddypress 1.5.1 using WordPress version 3.2.1, following the directions here. I’ve just uploaded the functions.php file with the code

if ( !function_exists( 'bp_dtheme_enqueue_styles' ) ) :
    function bp_dtheme_enqueue_styles() {}
endif;

as it says on that page, but I’m getting this error:

Fatal error: Cannot redeclare bp_dtheme_ajax_querystring() (previously declared in /home/sddia/public_html/wp-content/plugins/buddypress/bp-themes/bp-default/_inc/ajax.php:17) in /home/sddia/public_html/wp-content/plugins/buddypress/bp-themes/bp-default/_inc/ajax.php on line 81

Any idea what is going wrong here?