i'm using simple facebook connect only to provide a way for users to login using facebook. I've checked only the "Allow FB Login to Comment (for non-registered users)" fonctionnality.
The problem is, like a lot of people said before, sfc does call the_content filters to get the content to "publish to facebook". But i'm not using that feature, and my shortcodes/filters are still called twice by page.
I'll help you, it's n the file sfc-base.php, on line 489,
i commented "add_action('wp_head','sfc_base_meta');" and the user can still log in with facebook and the filters/shortcodes aren't processed twice.
You just need to add a check of the need to process the content.