Custom rewrite to page but no url substitution

Hi

I’ve got a strange problem. I have a page with follwing permalink
/my-first-page ( id = 20 )

Now i want to have another url /foo pointng to /my-first-page

So i did some research on that and found following solution

add_filter('rewrite_rules_array','wp_insertMyRewriteRules');
add_action('init','flushRules');

function flushRules(){
    global $wp_rewrite;
    $wp_rewrite->flush_rules();
}

function wp_insertMyRewriteRules($rules)
{
    $newrules = array();
    $newrules['^foo$'] = 'index.php?p=20';
    return $newrules + $rules;
}

Thats just working but wp redirects to the url /my-first-page.
I dont want wp to change my url rather i want to tell wp take this page id and show it under my custom url /foo