message: Ignore duplicate client sequence number (CSN) detection if existing CSN not defined.
  Used in back-compat scenarios since we may have checkpointed Deli without the CID.
diff: @@ -284,8 +284,9 @@ export class DeliLambda implements IPartitionLambda {
  clientSequenceNumber = message.operation.clientSequenceNumber;
  }
  + // TODO second check is to maintain back compat - can remove after deployment
  const node = this.clientNodeMap.get(clientId);
  - if (!node) {
  + if (!node || (node.value.clientSequenceNumber === undefined)) {
  return false;
  }
  