<?php
$items = $this->find();
$itemsToDuplicate = $items->filter(function ($item) {
   return $item->isDebit && $item->isCredit;
});
// debug(spl_object_hash($items->toArray()[0]));
// debug(spl_object_hash($itemsToDuplicate->toArray()[0]));
// they are the same

$items = $items->map(function ($item) {
   if ($item->isDebit) {
      $item->isCredit = false;
   }
   return $item;
});

$items = $items->append($itemsToDuplicate);
// $itemsToDuplicate is not appended