gistfile1.txt

<?php
function is_subcat( $term ) {
	global $wpdb;
	$check = $wpdb->get_var( "SELECT terms_id FROM " . $wpdb->prefix . "termsmeta WHERE meta_value = '" . $term . "' AND meta_key = 'non-unique-slug'" );
	if( $check == NULL ) {
		return false;
	} else {
		return true;
	}
}

function enable_non_unique_subcats( $query ) {	
	global $wpdb, $parent, $child;
	if( is_subcat($query->query_vars['attachment']) == true ) {
		$bits = explode('/', $_SERVER['REQUEST_URI'] );
		$c = array_pop( $bits );
		$p = array_pop( $bits );
		$parent = get_category_by_slug( $p );
		
		$ch = $wpdb->get_results( "SELECT terms_id FROM " . $wpdb->prefix . "termsmeta WHERE meta_value = '" . $parent->slug . "' AND meta_key = 'parent'" );
		foreach( $ch as $chld ) {
			$candidates = $wpdb->get_results( "SELECT terms_id, meta_value FROM " . $wpdb->prefix . "termsmeta WHERE terms_id = " . $chld->terms_id . " AND meta_key = 'non-unique-slug'" );
			foreach( $candidates as $candies ) {
				if( $candies->meta_value == $c ) {
					$child = get_category($candies->terms_id);
				}
			}
		}
		
		load_template( TEMPLATEPATH . '/subcat.php' );
	}
}

add_action( 'pre_get_posts', 'enable_non_unique_subcats' );
?>