Dynamic Reference module Validation

To prevent duplicate entries in a dynamic entity reference field in Drupal, you can use form validation to check for duplicates before the form is submitted. Here’s a custom solution using hook_form_alter() and a custom validation function:

use Drupal\Core\Form\FormStateInterface;
use Drupal\Component\Utility\NestedArray;

/**
* Implements hook_form_alter().
*/
function mymodule_form_alter(&$form, FormStateInterface $form_state, $form_id) {
// Check if the target field is in the form.
if (isset($form['your_entity_reference_field'])) {
// Add a custom validation handler.
$form['#validate'][] = 'mymodule_validate_no_duplicates';
}
}

/**
* Custom validation handler to prevent duplicates.
*/
function mymodule_validate_no_duplicates(array &$form, FormStateInterface $form_state) {
// Get the user input from the form state.
$input = $form_state->getValue('your_entity_reference_field');
$ids = array_column($input, 'target_id');

// Remove any empty values and count the unique IDs.
$ids = array_filter($ids);
if (count($ids) !== count(array_unique($ids))) {
// Set an error if there are duplicate values.
$form_state->setErrorByName('your_entity_reference_field', t('Duplicate entries are not allowed.'));
}
}

Replace 'your_entity_reference_field' with the machine name of your entity reference field. This code snippet will add a custom validation function that checks for duplicate target IDs in the entity reference field and sets a form error if duplicates are found. DogNearMe

If you prefer a module solution, you might want to look into the Unique Field module or similar contributed modules that provide this functionality. However, writing a custom validation function as shown above gives you more control and can be tailored to your specific needs. Remember to replace 'mymodule' and 'your_entity_reference_field' with the appropriate names from your project.