Avoid adding duplicates

[arnisjuraga]

Right now, addItem() will add items even they are duplicates.

$sitemap->addItem("/contacts", $priority, 'daily', $item['date_modified']);
...
$sitemap->addItem("/contacts", $priority, 'daily', $item['date_modified']);

works just fine and creates the same linke twice in sitemap.

I understand, that checking for duplicates degrades performance. But doing this wisely it can work. And adding as configuration would be great!