I am not aware of a dedicated action, but you could prevent double processing with an internal cache in your function.
Pseudo code:
function content_filter( $content )
{
static $cache = array();
$id = get_the_ID();
if ( in_array( $id, $cache ) )
{
return $content; // we did this already
}
$cache[] = $id;
return $content . 'your awesome extra';
}