SEO and code style question with hook_menu()

I'm building a module that adds a page with several tabs. Drupal requires that hook_menu define the "default" page twice, like this:

/**
 * Implementation of hook_menu().
 */
function user_deco_menu($may_cache) {
  $items = array();
  if (!$may_cache) {
    $items[] = array(
      'path' => 'user_deco',
      'title' => t('Decos'),
      'description' => t('Shows a gallery of user_decos with the option to buy them.'),
      'callback' => 'user_deco_gallery',
      'callback arguments' => 'gallery',
      'access' => user_access('buy user_decos'),
      'type' => MENU_NORMAL_ITEM,
    );
    $items[] = array(
      'path' => 'user_deco/gallery',
      'title' => t('Gallery'),
      'type' => MENU_DEFAULT_LOCAL_TASK,
      'weight' => -1,
    );
  }
  return $items;
}

The result is that there are two URLs to access the same page. (Same goes for node/* and node/*/view, and many other pages in the Drupal ecosystem.) So, two questions.

1. Is this a problem for SEO? I've read that Google, at least, does not penalize sites for having duplicate pages, so I doubt this is really an issue.
2. Which page should the module link to: user_deco or user_deco/gallery, in order to comply with Drupal coding standards?

In regards to the second question, Drupal links the tab to user_deco. Is this a case where I should do it as core does it?