How Do You Add a Second “Posts” Menu to Your Dashboard?

What you are looking for is called a Custom Post Type, or CPT in short term. To register a new post type, you can use a simple code like this, which will add a new post type called article to your admin panel:

function register_my_post_type() {
    $labels = array(
        'name'                => __( 'Articles', 'text-domain' ),
        'singular_name'       => __( 'Article', 'text-domain' ),
        'menu_name'           => __( 'Articles', 'text-domain' ),
        'parent_item_colon'   => __( 'Parent Article', 'text-domain' ),
        'all_items'           => __( 'All Articles', 'text-domain' ),
        'view_item'           => __( 'View Article', 'text-domain' ),
        'add_new_item'        => __( 'Add New Article', 'text-domain' ),
        'add_new'             => __( 'Add New', 'text-domain' ),
        'edit_item'           => __( 'Edit Article', 'text-domain' ),
        'update_item'         => __( 'Update Article', 'text-domain' ),
        'search_items'        => __( 'Search Artcile', 'text-domain' ),
        'not_found'           => __( 'Not Found', 'text-domain' ),
        'not_found_in_trash'  => __( 'Not found in Trash', 'text-domain' ),
    );  
    $args = array(
        'label'               => __( 'articles', 'text-domain' ),
        'description'         => __( 'Website\'s articles', 'text-domain' ),
        'labels'              => $labels,
        'supports'            => array( 'title', 'editor', 'excerpt', 'author', 'thumbnail', 'comments', 'revisions', 'custom-fields', ),
        'hierarchical'        => false,
        'public'              => true,
        'show_ui'             => true,
        'show_in_menu'        => true,
        'show_in_nav_menus'   => true,
        'show_in_admin_bar'   => true,
        'menu_position'       => 5,
        'can_export'          => true,
        'has_archive'         => true,
        'exclude_from_search' => false,
        'publicly_queryable'  => true,
        'capability_type'     => 'post',        
        'taxonomies'          => array( 'category','post_tag' ),
    );
    register_post_type( 'article', $args );
}
add_action( 'init', 'register_my_post_type', 0 );

The first array includes the names and titles for various part of your post type, such as singular/plural title, menu’s name, and so on.

The second array includes the features such as being included in search, being public, and so on.

For a more detailed information about this, check the official codex page about register_post_type.