duplicate posts with ajax load more wordpress

I have ajax loading posts on my site when scrolling. The problem lies in the appearance of duplicate posts. I cannot understand why this is happening. Only some posts have duplicates. Here is my code in function.php:

function loadmore_get_posts(){
  $paged = !empty($_POST['paged']) ? $_POST['paged'] : 1;
  $paged++;

  $args = array(
    'paged' => $paged,
    'posts_per_page' => $_POST['posts_per_page'],
    'post_type'      => 'post',
    'post_status' => 'publish',
    'cat' => $_POST['cats']
    );

  query_posts($args);
 
  while( have_posts() ) : the_post();
    get_template_part( 'this is template' );    
  endwhile;
  die;
}
add_action('wp_ajax_loadmore', 'loadmore_get_posts');
add_action('wp_ajax_nopriv_loadmore', 'loadmore_get_posts');

And this is ajax in vanilla javascript:

let ajaxurl = '<?php echo admin_url('admin-ajax.php') ?>';
let section_posts = 1;
let postData = new FormData();

postData.append('action', 'loadmore');
postData.append('paged', section_posts);
postData.append('posts_per_page', 9);
postData.append('cats',  <?php print json_encode(get_selected_cats())?>);

  const xhr = new XMLHttpRequest();
  xhr.open('POST', ajaxurl);

  xhr.addEventListener('readystatechange', function (data) {
  if (this.readyState === 4 && this.status === 200) {
     section_posts++;
     document.querySelector('.articlefeed_template').innerHTML += data.target.responseText;
  } else {}
     xhr.send(postData);
  }

Please, tell me what I am wrong about? Thanks!