see https://forum.matomo.org/t/error-upgrading-db-while-upgrading-to-4-0-0-b1/39478

• /opt/matomo/core/Updates/4.0.0-b1.php:
  Error trying to execute the migration 'ALTER TABLE matomo_session CHANGE id id VARCHAR(191);'.
  The error was: SQLSTATE[23000]: Integrity constraint violation: 1062 Duplicate entry 'AQBqYXZhLnV0aWwuVHJlZU1h8AEBAW9yZy5hcGFjaGUuY29tbW9ucy5iZWFud...' for key 'PRIMARY'