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::VSTR, UniValue::VARR}, true);
    
    PartiallySignedTransaction psbtx;
    std::string error;
    if (!DecodeBase64PSBT(psbtx, request.params[0].get_str(), error)) {
        throw JSONRPCError(RPC_DESERIALIZATION_ERROR, strprintf("TX decode failed %s", error));
    }
    
    FlatSigningProvider provider;
    if (!request.params[1].isNull()) {
        auto descs = request.params[1].get_array();
        for (size_t i = 0; i < descs.size(); ++i) {
            EvalDescriptorStringOrObject(descs[i], provider);
        }
    }
    
    HidingSigningProvider public_provider(&provider,  true,  false);
    
    CCoinsView viewDummy;
    CCoinsViewCache view(&viewDummy);
    {
        const CTxMemPool& mempool = EnsureMemPool(request.context);
        LOCK2(cs_main, mempool.cs);
        CCoinsViewCache &viewChain = ::ChainstateActive().CoinsTip();
        CCoinsViewMemPool viewMempool(&viewChain, mempool);
        view.SetBackend(viewMempool); 
        for (const CTxIn& txin : psbtx.tx->vin) {