class UniqueFieldValueValidator

class UniqueFieldValueValidator extends ConstraintValidator implements ContainerInjectionInterface {
    
    /**
     * Creates a UniqueFieldValueValidator object.
     *
     * @param \Drupal\Core\Entity\EntityFieldManagerInterface $entityFieldManager
     *   The entity type manager.
     * @param \Drupal\Core\Entity\EntityTypeManagerInterface $entityTypeManager
     *   The entity type manager.
     */
    public function __construct(EntityFieldManagerInterface $entityFieldManager, EntityTypeManagerInterface $entityTypeManager) {
    }
    
    /**
     * {@inheritdoc}
     */
    public static function create(ContainerInterface $container) {
        return new static($container->get('entity_field.manager'), $container->get('entity_type.manager'));
    }
    
    /**
     * {@inheritdoc}
     */
    public function validate($items, Constraint $constraint) : void {
        if (!$items->first()) {
            return;
        }
        
        /** @var \Drupal\Core\Entity\EntityInterface $entity */
        $entity = $items->getEntity();
        $entity_type = $entity->getEntityType();
        $entity_type_id = $entity_type->id();
        $entity_label = $entity->getEntityType()
            ->getSingularLabel();
        $field_name = $items->getFieldDefinition()
            ->getName();
        $field_label = $items->getFieldDefinition()
            ->getLabel();
        $field_storage_definitions = $this->entityFieldManager
            ->getFieldStorageDefinitions($entity_type_id);
        $property_name = $field_storage_definitions[$field_name]->getMainPropertyName();
        $id_key = $entity_type->getKey('id');
        $is_multiple = $field_storage_definitions[$field_name]->isMultiple();
        $is_new = $entity->isNew();
        $item_values = array_column($items->getValue(), $property_name);
        // Check if any item values for this field already exist in other entities.
        $query = $this->entityTypeManager
            ->getStorage($entity_type_id)
            ->getAggregateQuery()
            ->accessCheck(FALSE)
            ->condition($field_name, $item_values, 'IN')
            ->groupBy("{$field_name}.{$property_name}");
        if (!$is_new) {
            $entity_id = $entity->id();
            $query->condition($id_key, $entity_id, '<>');
        }
        $results = $query->execute();
        if (!empty($results)) {
            // The results array is a single-column multidimensional array. The
            // column key includes the field name but may or may not include the
            // property name. Pop the column key from the first result to be sure.
            $column_key = key(reset($results));
            $other_entity_values = array_column($results, $column_key);
            // If our entity duplicates field values in any other entity, the query
            // will return all field values that belong to those entities. Narrow
            // down to only the specific duplicate values.
            $duplicate_values = $this->caseInsensitiveArrayIntersect($item_values, $other_entity_values);
            foreach ($duplicate_values as $delta => $dupe) {
                $violation = $this->context
                    ->buildViolation($constraint->message)
                    ->setParameter('@entity_type', $entity_label)
                    ->setParameter('@field_name', $field_label)
                    ->setParameter('%value', $dupe);
                if ($is_multiple) {
                    $violation->atPath((string) $delta);
                }
                $violation->addViolation();
            }
        }
        // Check if items are duplicated within this entity.
        if ($is_multiple) {
            $duplicate_values = $this->extractDuplicates($item_values);
            foreach ($duplicate_values as $delta => $dupe) {
                $this->context
                    ->buildViolation($constraint->message)
                    ->setParameter('@entity_type', $entity_label)
                    ->setParameter('@field_name', $field_label)
                    ->setParameter('%value', $dupe)
                    ->atPath((string) $delta)
                    ->addViolation();
            }
        }
    }
    
    /**
     * Perform a case-insensitive array intersection, but keep original capitalization.
     *
     * @param array $orig_values
     *   The original values to be returned.
     * @param array $comp_values
     *   The values to intersect $orig_values with.
     *
     * @return array
     *   Elements of $orig_values contained in $comp_values when ignoring capitalization.
     */
    private function caseInsensitiveArrayIntersect(array $orig_values, array $comp_values) : array {
        $lowercase_comp_values = array_map('strtolower', $comp_values);
        $intersect_map = array_map(fn(string $x) => in_array(strtolower($x), $lowercase_comp_values, TRUE) ? $x : NULL, $orig_values);
        return array_filter($intersect_map, function ($x) {
            return $x !== NULL;
        });
    }
    
    /**
     * Get an array of duplicate field values.
     *
     * @param array $item_values
     *   The item values.
     *
     * @return array
     *   Item values only for deltas that duplicate an earlier delta.
     */
    private function extractDuplicates(array $item_values) : array {
        $value_frequency = array_count_values($item_values);
        // Filter out item values which are not duplicates while preserving deltas
        $duplicate_values = array_intersect($item_values, array_keys(array_filter($value_frequency, function ($value) {
            return $value > 1;
        })));
        // Exclude the first delta of each duplicate value.
        $first_deltas = array_unique($duplicate_values);
        return array_diff_key($duplicate_values, $first_deltas);
    }

}