Multiple instances of the shortcode on the same page

I have this shortcode:

function fm_requestebook ($atts) {
    $atts = shortcode_atts(array(
            'cf7'           =>  null,
            'filename'      =>  null,
            'event'         =>  null,
            'testo'         =>  "Scarica eBook"
             ), $atts);

    $contact_form = do_shortcode("[contact-form-7 id=".$atts['cf7']."]");
    $hf = '<input type="hidden" name="_FM_ebook" value="' . $atts["filename"]. '" /></form>';
    $contact_form = str_replace("</form>", $hf, $contact_form);
    $return='<a onclick="ga(\'send\',\'event\', \'Download\', \'ebook\', \''.$atts["event"].'\');" class="fancybox" href="#ebook" target="_blank" rel="nofollow"><span class="red-button">'.$atts["testo"].'</span></a><div class="fancybox-hidden" style="display: none;"><div id="ebook" class="hentry">'.$contact_form.'</div></div>';

    return do_shortcode($return);

}
add_shortcode ("request-ebook","fm_requestebook");

The problem is that if I put more than one instance of the shortcode on the same page, all the subsequent codes will have the same "_FM_ebook" value....