Jehu

In 3.5.2, the filters work by calling each of the defined functions in turn, passing the current result each time wp-includes\plugin.php

do {
		foreach( (array) current($wp_filter[$tag]) as $the_ )
			if ( !is_null($the_['function']) ){
				$args[1] = $value;
				$value = call_user_func_array($the_['function'], array_slice($args, 1, (int) $the_['accepted_args']));
			}

	} while ( next($wp_filter[$tag]) !== false );

By appending to the current result you’ll not overwrite existing values

function ald_more_reccurences( $schedules ) {
/*	return array(
		'weekly' => array('interval' => 604800, 'display' => __( 'Once Weekly', TPTN_LOCAL_NAME )),
		'fortnightly' => array('interval' => 1209600, 'display' => __( 'Once Fortnightly', TPTN_LOCAL_NAME )),
		'monthly' => array('interval' => 2419200, 'display' => __( 'Once Monthly', TPTN_LOCAL_NAME )),
);
*/
	$schedules['weekly'] = array('interval' => 604800, 'display' => __( 'Once Weekly', TPTN_LOCAL_NAME ));
	$schedules['fortnightly'] = array('interval' => 1209600, 'display' => __( 'Once Fortnightly', TPTN_LOCAL_NAME ));
	$schedules['monthly'] = array('interval' => 2419200, 'display' => __( 'Once Monthly', TPTN_LOCAL_NAME ));

	return $schedules;

}

The plugin prefixes the key of the schedules it adds with hmbkp_ so that when it retrieves the schedules it can filter to the ones it uses.

/**
	 * Return an array of BackUpWordPress cron schedules
	 *
	 * @return array
	 */
	public function get_cron_schedules(){

		$schedules = wp_get_schedules();

		// remove any schedule whose key is not prefixed with 'hmbkp_'
		foreach ( $schedules as $key => $arr ) {
			if( ! preg_match("/^hmbkp_/", $key ) )
				unset( $schedules[$key] );
		}

		return $schedules;
	}