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.

RPCResult::Type::STR, "", "The hex-encoded raw transaction with signature(s)"
                },
                RPCExamples{
                    HelpExampleCli("combinerawtransaction", R"('["myhex1", "myhex2", "myhex3"]')")
                },
        [&](const RPCHelpMan& self, const JSONRPCRequest& request) -> UniValue
{
    UniValue txs = request.params[0].get_array();
    std::vector<CMutableTransaction> txVariants(txs.size());
    for (unsigned int idx = 0; idx < txs.size(); idx++) {
        if (!DecodeHexTx(txVariants[idx], txs[idx].get_str())) {
            throw JSONRPCError(RPC_DESERIALIZATION_ERROR, strprintf("TX decode failed for tx %d. Make sure the tx has at least one input.", idx));
        }
    }
    if (txVariants.empty()) {
        throw JSONRPCError(RPC_DESERIALIZATION_ERROR, "Missing transactions");
    }
    
    
    CMutableTransaction mergedTx(txVariants[0]);
    
    CCoinsView viewDummy;
    CCoinsViewCache view(&viewDummy);
    {
        const CTxMemPool& mempool = EnsureMemPool(request.context);
        LOCK(cs_main);
        LOCK(mempool.cs);
        CCoinsViewCache &viewChain = ::ChainstateActive().CoinsTip();
        CCoinsViewMemPool viewMempool(&viewChain, mempool);
        view.SetBackend(viewMempool);