<?php
function clean($full_url){
    $parts = explode('//', $full_url, 2);
    $parts[1] = rtrim(preg_replace('@/+@', '/', $parts[1]), '/');
    $full_url = implode('//', $parts);
    return $full_url;
}

$urls = array( 'http://stackoverflow.com/questions/13990256/remove-duplicate-trailing-slashes/',
 'http://stackoverflow.com/questions/13990256/remove-duplicate-trailing-slashes////',
 'http://stackoverflow.com///questions///13990256///remove-duplicate-trailing-slashes////',
 'http://stackoverflow.com/questions//13990256/remove-duplicate-trailing-slashes//');
 
 foreach($urls as $url){
     echo "Before: $url\nAfter:  ".clean($url)."\n---------------------\n";
 }
 ?>