The NodeJS client library automatically retries transactions that return an Unknown error. This is because a transaction in some (very few) cases can return Transaction Outcome Unknown. This error is however not an indication that the transaction for certain did not succeed. As the error message indicates, the outcome is unknown and it is possible that the transaction succeeded. The application needs to take additional steps to check whether the transaction actually succeeded or not, and then if necessary manually retry the transaction.
The automatic retry of these errors should therefore be removed from the retry logic in the client library.