Sorry, I got it to works, I change
"cat" => "$category_id" by "category__in" => array(get_query_var("cat"))
<?php $current_category = single_cat_title("", false); $category_id = get_cat_ID($current_category); $cat_posts = new WP_Query(
array( "showposts" => "1",
"tag" => "principal",
"category__in" => array(get_query_var("cat")),
) );
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