A Symphony Event (for 2.3x) which looks at the query string produced from a front-end form submission using a GET request and for any keys which have multiple values it wraps them up into a single XML element in the parameter pool for use within datasources. The numeric ones are left in place.

sym-merge-get-param-arrays.php

<?php
 
require_once(TOOLKIT.'/class.event.php');
 
Class eventmerge_get_param_arrays extends Event {
 
    public $eParamFILTERS = array(
 
    );
 
    public static function about() {
        return array(
                'name' =>'Merge $_GET param arrays',
                'author' => array(  
                    'name' => 'Ian Young',
                    'website' => 'http://',
                    'email' => 'i.young@me.com'
                ),
                'version' => '0.1',
                'release-date' => '2013-08-23'
        );
    }
 
    public static function allowEditorToParse() {
        return false;
    }
 
    public static function documentation() {
        return '';
    }
 
    public function load() {
        $trigger = false;
        foreach($_GET as $getParam) {
            if (is_array($getParam)) $trigger = true;
        }
        if ($trigger) return $this -> __trigger();
    }

    protected function __trigger(){
        $page = Frontend::instance()->Page();
        if(is_array($_GET) && !empty($_GET)){
            foreach($_GET as $key => $value) {
                if(is_array($value)){
                    $page->_param['url-'.$key] = join(',', $value);
                }
            }
        }
    }
 
}