Create unique field in a content type

If you want to add a unique field to content type so that Drupal would prevent you from entering the same field value more than once you can add a validation constraint to your field. There is two ways to do that:

1. Setting a validation constraint to a field you haven't defined by implementing hook_entity_bundle_field_info_alter:

   function YOURMODULE_entity_bundle_field_info_alter(&$fields, EntityTypeInterface $entity_type, $bundle) {
     if ($entity_type->id() === 'ENTITY_TYPE' && $bundle === 'BUNDLE_NAME') {
       if (isset($fields['FIELD_NAME'])) {
         $fields['FIELD_NAME']->addConstraint('UniqueField');
       }
     }
   }

   
    
2. Setting a validation constraint to a field you have defined in baseFieldDefinitions:

   public static function baseFieldDefinitions(EntityTypeInterface $entityType) {
    $fields['FIELD_NAME'] = BaseFieldDefinition::create('string')
      ->setLabel(t('MY UNIQUE FIELD'))
      ->addConstraint('UniqueField');
   
    return $fields;
   }