Please beware that Duplicate This Post duplicates everything about the post, including the GUID. The only problem is that the Globally Unique IDentifier (GUID) is no longer unique once it is duplicated. That can cause trouble if your posts appear in feeds.
To resolve this you can patch line 86 of class/mm-duplicate.php:
$post_data['guid'] = $post->guid;
to instead be:
$post_data['guid'] = uniqid($post->guid . ' ');
Perhaps that code could end up in the next version.