Try increasing both of them.
Perhaps 200 for the Max Results and 60 for the Max Execution Time.
The first one should reduce the number of requests sent to Google so they are less likely to throttle? It will put a bigger load on your importing server though.
The function that checks for duplicate comments is as follows:
function comment_exists($post_id, $comment_author, $comment_date)
$comment_author = stripslashes($comment_author);
$comment_date = stripslashes($comment_date);
return $wpdb->get_var($wpdb->prepare("SELECT comment_ID FROM $wpdb->comments WHERE comment_post_ID = %s and comment_author = %s AND comment_date = %s", $post_id, $comment_author, $comment_date));
So unless the date or author has changed somehow I can't see why you'd be getting duplicates.