Hook for Field Duplication
You want hook_field_formatter_prepare_view() instead. Note this hook operate on a group of entities, not on a single one.

Here's a working snippet. Some array-fu is needed to not end up writing an ugly loop of ten lines, since deltas must be preserved linear.

&lt;?php

function MODULE_field_formatter_prepare_view($entity_type, $entities, $field, $instances, $langcode, &amp;$items, $displays) {
  foreach ($entities as $id =&gt; $entity) {
    $array = array(0 =&gt; $items[$id][0], 1 =&gt; $items[$id][0]);
    if (count($items[$id]) &gt; 1) {
      array_splice($array, 2, 0, array_slice($items[$id], 1));
    }
    $items[$id] = $array;
  }
}