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.

for (const CTxIn& txin : mergedTx.vin) {
            view.AccessCoin(txin.prevout); 
        }
        view.SetBackend(viewDummy); 
    }
    
    
    const CTransaction txConst(mergedTx);
    
    for (unsigned int i = 0; i < mergedTx.vin.size(); i++) {
        CTxIn& txin = mergedTx.vin[i];
        const Coin& coin = view.AccessCoin(txin.prevout);
        if (coin.IsSpent()) {
            throw JSONRPCError(RPC_VERIFY_ERROR, "Input not found or already spent");
        }
        SignatureData sigdata;
        
        for (const CMutableTransaction& txv : txVariants) {
            if (txv.vin.size() > i) {
                sigdata.MergeSignatureData(DataFromTransaction(txv, i, coin.out));
            }
        }
        ProduceSignature(DUMMY_SIGNING_PROVIDER, MutableTransactionSignatureCreator(&mergedTx, i, coin.out.nValue, 1), coin.out.scriptPubKey, sigdata);
        UpdateInput(txin, sigdata);
    }
    return EncodeHexTx(CTransaction(mergedTx));
},
    };
}
static RPCHelpMan signrawtransactionwithkey()