At the moment, EthSigner has a "NonceTooLow" retry mechanism, such that if a PUBLIC transaction failed due to a reported error of "Nonce Too Low", a new nonce would be determined and the transaction would be resubmitted.
Unfortunately, this retryMechanism looks SPECIFICALLY for an error code of NONCE_TOO_LOW - this does not work for the current implementation of EeaSendRawPrivateTransaction - which responds with an error code of PRIVATE_NONCE_TOO_LOW.
There are two potential solutions:
1. Update EthSIgner to look for a PRIVATE_NONCE_TOO_LOW when performing PrivateTransactions
2. Update Pantheon Private Transation Validator to re-use NONCE_TOO_LOW error code for Private Transactions (rather than having a custom error code).
There are pros-and-cons for both technical solutions - complexity is SIMILAR in both cases - possibly less than 1 day.