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.

+ HelpExampleRpc("sendrawtransaction", "\"signedhex\"")
                },
        [&](const RPCHelpMan& self, const JSONRPCRequest& request) -> UniValue
{
    RPCTypeCheck(request.params, {
        UniValue::VSTR,
        UniValueType(), 
    });
    CMutableTransaction mtx;
    if (!DecodeHexTx(mtx, request.params[0].get_str())) {
        throw JSONRPCError(RPC_DESERIALIZATION_ERROR, "TX decode failed. Make sure the tx has at least one input.");
    }
    CTransactionRef tx(MakeTransactionRef(std::move(mtx)));
    const CFeeRate max_raw_tx_fee_rate = request.params[1].isNull() ?
                                             DEFAULT_MAX_RAW_TX_FEE_RATE :
                                             CFeeRate(AmountFromValue(request.params[1]));
    int64_t virtual_size = GetVirtualTransactionSize(*tx);
    CAmount max_raw_tx_fee = max_raw_tx_fee_rate.GetFee(virtual_size);
    std::string err_string;
    AssertLockNotHeld(cs_main);
    NodeContext& node = EnsureNodeContext(request.context);
    const TransactionError err = BroadcastTransaction(node, tx, err_string, max_raw_tx_fee,  true,  true);
    if (TransactionError::OK != err) {
        throw JSONRPCTransactionError(err, err_string);
    }
    return tx->GetHash().GetHex();
},
    };
}
static RPCHelpMan testmempoolaccept()