Migration runs twice, updates content second time through

I have a fairly basic D6->D7 migration set up using migrate_d2d, and I"m having an issue with my node migrations where the migration runs twice. It will complete the migration once, stop (or do something else, I don't know what), and then re-run with the same nodes again, updating them. The final output from the drush mi command will be

Processed 4 (2 created, 2 updated, 0 failed, 0 ignored) in 98.7 sec (2/min) - done with 'CNSArticle'           

when I've run it for two nodes using the --idlist parameter. It didn't do this with user, taxonomy, and file migrations, and the node migrations are fairly straightforward, with no additions or overrides of the query.

Here's the code to set up the migrations:

/**
 * Register all D6->D7 migrations.
 */
function cns_migrate_register_migrations() {
  // Define arguments used for all migrations.
  $common_arguments = array(
    'source_connection' => 'legacy',
    'source_version' => 6,
    'format_mappings' => array(
      1 => 'filtered_html',
      2 => 'full_html',
      3 => 'full_html',
      4 => 'full_html',
    ),
  );

  $node_arguments = array(
    array(
      'class_name' => 'CNSArticleMigration',
      'description' => t('Migration of CNS article nodes from Drupal 6'),
      'machine_name' => 'CNSArticle',
      'source_type' => 'article',
      'destination_type' => 'article',
      'dependencies' => array('Category', 'Tags'),
    ),
    array(
      'class_name' => 'APArticleMigration',
      'description' => t('Migration of AP article nodes from Drupal 6'),
      'machine_name' => 'APArticle',
      'source_type' => 'articleap',
      'destination_type' => 'articleap',
      'dependencies' => array('Category', 'Tags'),
    ),
  );
  // Tell the node migrations where the users are coming from, so they can
  // set up the dependency and resolve D6->D7 uids.
  $common_node_arguments = $common_arguments + array(
      'user_migration' => 'CNSUser',
      'group_name' => 'node',
    );
  foreach ($node_arguments as $arguments) {
    $arguments = array_merge_recursive($arguments, $common_node_arguments);
    MigrationBase::registerMigration($arguments['class_name'], $arguments['machine_name'],
      $arguments);
  }
} 

And the classes themselves:

class ArticleMigration extends DrupalNode6Migration {
  public function __construct(array $arguments) {
    parent::__construct($arguments);

    // Mappings for taxonomy terms.
    $this->addFieldMapping('field_category', 1)
      ->sourceMigration('Category');
    $this->addFieldMapping('field_category:source_type')->defaultValue('tid');
    $this->addFieldMapping('field_tags', 2)
      ->sourceMigration('Tags');
    $this->addFieldMapping('field_tags:source_type')->defaultValue('tid');
    // Images multifield
    $this->addFieldMapping('field_images:field_image', 'field_image')
      ->sourceMigration('CNSFile');
    $this->addFieldMapping('field_images:field_image:file_class')
      ->defaultValue('MigrateFileFid');
    $this->addFieldMapping('field_images:field_image:preserve_files')
      ->defaultValue(TRUE);
  }

  public function prepare($entity, stdClass $row) {
    // Add source nid to Disqus field so that comments are mapped to new nodes.
    // Uses patch from https://www.drupal.org/node/1175668#comment-7847119.
    // nid 77900 is when Disqus comments were first used.
    if ($row->nid > 77900) {
      $entity->disqus = array(
        'domain' => variable_get('disqus_domain', ''),
        'url' => $entity->path['alias'],
        'title' => $entity->title,
        'did' => NULL,
        'developer' => 1,
      );

      $entity->disqus_identifier_auto = FALSE;
      $entity->disqus_status = TRUE;
      $entity->disqus_identifier = 'node/' . $row->nid;
    }
    else {
      // Pre Disqus, so we disable Disqus on the node.
      $entity->disqus_status = FALSE;
      $entity->disqus_identifier_auto = FALSE;
    }
  }
}

class CNSArticleMigration extends ArticleMigration {
  public function __construct(array $arguments) {
    parent::__construct($arguments);

    // Author
    $this->addFieldMapping('field_author', 'field_source')
      ->sourceMigration('CNSUser');
  }
}

class APArticleMigration extends ArticleMigration {
  public function __construct(array $arguments) {
    parent::__construct($arguments);

    // Author
    $this->addFieldMapping('field_author_ap', 'field_apauthor');
    // AP tracking fields.
    $this->addSimpleMappings(array('field_ap_mgmt_id', 'field_ap_mgmt_seq_num'));

  }
}

This also happens if I use the CNSArticleMigration and APArticleMigration classes directly instead of deriving from the ArticleMigration class. I also thought that the multifield migrate handler (something I'm having a separate issue with) might be the cause, but when I remove that, I still have this problem.

I've googled around and haven't been able to find a similar issue. Anyone know what could be causing this? I'm not sure if it's particular to migrate_d2d or migrate, so I'm starting here.

Thanks.