Duplicate nodes, distinct doesn't fix it

After an upgrade from 4.7 to 5.5 I've upgraded views to 1.6, deleted all caches, resubmitted the modules page and still run into this problem.

I have a A/B/C style taxonomy block and all B-nodes are shown in duplicates. See http://55.figu.org/ch/themen_von_a_bis_z/D. I am pulling my hair out since I already tried all I know (setting up a new view etc.). If I use distinct (as being told once with 4.7), all A-nodes are duplicate as well.

Here's the view:

  $view = new stdClass();
  $view->name = 'autoren';
  $view->description = 'Autoren A/B/C';
  $view->access = array ();
  $view->view_args_php = '';
  $view->page = TRUE;
  $view->page_title = 'Alle Autoren von A bis Z';
  $view->page_header = '';
  $view->page_header_format = '1';
  $view->page_footer = '';
  $view->page_footer_format = '1';
  $view->page_empty = '';
  $view->page_empty_format = '1';
  $view->page_type = 'list';
  $view->url = 'autoren_von_a_bis_z';
  $view->use_pager = TRUE;
  $view->nodes_per_page = '30';
  $view->sort = array (
    array (
      'tablename' => 'term_data',
      'field' => 'weight',
      'sortorder' => 'ASC',
      'options' => '',
    ),
    array (
      'tablename' => 'node',
      'field' => 'title',
      'sortorder' => 'ASC',
      'options' => '',
    ),
  );
  $view->argument = array (
    array (
      'type' => 'taxletter',
      'argdefault' => '4',
      'title' => 'Buchstabe \'%1\'',
      'options' => '1',
      'wildcard' => '',
      'wildcard_substitution' => '',
    ),
    array (
      'type' => 'taxletter',
      'argdefault' => '6',
      'title' => 'Alle Artikel von \'%2\'',
      'options' => '',
      'wildcard' => '',
      'wildcard_substitution' => '',
    ),
  );
  $view->field = array (
    array (
      'tablename' => 'node',
      'field' => 'title',
      'label' => '',
      'handler' => 'views_handler_field_nodelink',
      'defaultsort' => 'ASC',
      'options' => 'link',
    ),
  );
  $view->filter = array (
    array (
      'tablename' => 'node',
      'field' => 'status',
      'operator' => '=',
      'options' => '',
      'value' => '1',
    ),
    array (
      'tablename' => 'term_data',
      'field' => 'vid',
      'operator' => 'OR',
      'options' => '',
      'value' => array (
  0 => '8',
),
    ),
  );
  $view->exposed_filter = array (
  );
  $view->requires = array(term_data, node);
  $views[$view->name] = $view;

Any help is highly appreciated.
Thanks,
Stephan