gistfile1.txt

 public function validate($item, Constraint $constraint) {

    if ($item->isEmpty()) {
      return NULL;
    }

    $entity = $item->getEntity();
    $entity_id = $entity->id();
    $entity_type = $entity->getEntityType();
    $field_name = $item->getFieldDefinition()->getName();
    $properties = $item->getProperties();

    // Query to see if existing entity with machine name exists.
    $query = \Drupal::entityQuery($entity_type->id());

    foreach ($properties as $property) {
      $query->condition($field_name . '.value', $property->getValue());

      if (!empty($entity_id)) {
        $query->condition('entity.id', $entity_id, '<>');
      }
      $result = $query->execute();
      if (count($result) > 0) {
        $this->context->addViolation($constraint->message, array('%entity_type' => $entity_type->id(), '%value' => $property->getValue()));
      }
    }
  }