You should use post__not_in
parameter (see Post & Page Parameters in Codex). Since it takes an array you will also need to switch to array rather than string for passing arguments.
Something like this:
query_posts(array(
'showposts' => 1,
'cat' => get_catId( $instance['catOption4'] ),
'post__not_in' => $themename_do_not_duplicate,
));