add_menu_item() creating an extra menu

Hi All,
I’m developing a directory plugin to keep track of stores for a shopping center. I created a top-level admin menu for my plugin with the following code:

add_menu_page('Directory','Directory','edit_posts','directory66/admin-list.php','directory66_list');

The problem is, it automatically creates a sub-menu item with the same name. I don’t see that happening in other plugins, and I’d like to change that behavior.

Here is a picture of the admin menu which may make things clearer
http://testing.studio66design.com/menu.png

Thanks in advance for the help.