I discovered a race condition in sendrawtransaction. For details: see my comment in https://github.com/bitcoin/bitcoin/commit/1d46fe3327f6645fd79b442cd72ef422418c1a50.

If you send a raw transaction which has already been broadcasted, then you don't know whether you'll receive an RPC_TRANSACTION_ALREADY_IN_CHAIN exception, since you don't know whether a new block has just been received, which includes the transaction. There are use cases where such RPC_TRANSACTION_ALREADY_IN_CHAIN occurrences do not indicate an error; however, since this type of occurrence is very rare, developers are not likely to be aware of the possibility, and silence the exception. As a result, applications can potentially experience hard-to-diagnose, very rare errors.

I think the core of the problem is that sendrawtransaction tries to combine two use cases:
A: publish a transaction on the Bitcoin network
B: re-transmit an already known transaction to peers
RPC_TRANSACTION_ALREADY_IN_CHAIN might make sense for B, but for A (which is, IMHO, the primary use case) it is confusing.

},
        [&](const RPCHelpMan& self, const JSONRPCRequest& request) -> UniValue
{
    RPCTypeCheck(request.params, {UniValue::VARR}, true);
    
    std::vector<PartiallySignedTransaction> psbtxs;
    UniValue txs = request.params[0].get_array();
    if (txs.empty()) {
        throw JSONRPCError(RPC_INVALID_PARAMETER, "Parameter 'txs' cannot be empty");
    }
    for (unsigned int i = 0; i < txs.size(); ++i) {
        PartiallySignedTransaction psbtx;
        std::string error;
        if (!DecodeBase64PSBT(psbtx, txs[i].get_str(), error)) {
            throw JSONRPCError(RPC_DESERIALIZATION_ERROR, strprintf("TX decode failed %s", error));
        }
        psbtxs.push_back(psbtx);
    }
    PartiallySignedTransaction merged_psbt;
    const TransactionError error = CombinePSBTs(merged_psbt, psbtxs);
    if (error != TransactionError::OK) {
        throw JSONRPCTransactionError(error);
    }
    CDataStream ssTx(SER_NETWORK, PROTOCOL_VERSION);
    ssTx << merged_psbt;
    return EncodeBase64(MakeUCharSpan(ssTx));
},
    };
}
static RPCHelpMan finalizepsbt()