Simple script for copying fields from one bundle to another.

copy_field.php

<?php
$entity_type = '';
$old_bundle = '';
$new_bundle = '';

$fields = array(
  'field_name',
);

foreach ($fields as $field) {
  $info = field_info_instance($entity_type, $field, $old_bundle);
  $info_new = field_info_instance($entity_type, $field, $new_bundle);
  if ($info && !$info_new) {
    $info['bundle'] = $new_bundle;
    unset($info['id']);
    unset($info['field_id']);
    field_create_instance($info);
    drupal_set_message(t('Created instance of !field for !bundle', array('!field' => $field, '!bundle' => $new_bundle)));
  }
}
?>