Creating a Field Type Plugin but getting PHP Fatal Error: Cannot declare class ..., because the name is already in use

Hi jaypan, thanks for the reply, that seems to have addressed the error but I'm a little unsure as to why that would fix this specific error, since the error seems to be generated by PHP itself and I can't find any PHP documentation around that error which would suggest it must comply with any particular naming scheme. Of course my assumption is Drupal is doing something that is resulting in this error to be thrown when the namespace doesn't match what is expected but it seems a little strange. Are you or anyone else able to provide further clarification?

In addition I'm now getting a message when I try to create a new field of this type "There was a problem creating field Cerebro Item ID: The "cerebro_item_link_widget" plugin does not exist." But from what I can tell I've setup the plugin correctly based on the documentation, here is a copy of the file

namespace Drupal\cerebro_item_link\Plugin\Field\FieldWidget;

use Drupal\Core\Field\FieldItemListInterface;
use Drupal\Core\Field\WidgetBase;
use Drupal\Core\Form\FormStateInterface;

/**
 * Plugin implementation for the Cerebro Item Link widget.
 *
 * @FieldWidget(
 *   id = "cerebro_item_link",
 *   module = "cerebro_item_link",
 *   label = @Translation("Item ID"),
 *   field_types = {
 *     "cerebro_item_link"
 *   }
 * )
 */
class CerebroItemLinkWidget extends WidgetBase implements WidgetInterface {

  /**
   * {@inheritdoc}
   */
  public function formElement(FieldItemListInterface $items, $delta, array $element, array &$form, FormStateInterface $form_state) {
    // code here
  }

}

I've also tried setting the id to "cerebro_item_link_widget" but same problem, any help would be appreciated.

Thanks
Aaron