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.

in.pushKV("unknown", unknowns);
        }
        inputs.push_back(in);
    }
    result.pushKV("inputs", inputs);
    
    CAmount output_value = 0;
    UniValue outputs(UniValue::VARR);
    for (unsigned int i = 0; i < psbtx.outputs.size(); ++i) {
        const PSBTOutput& output = psbtx.outputs[i];
        UniValue out(UniValue::VOBJ);
        
        if (!output.redeem_script.empty()) {
            UniValue r(UniValue::VOBJ);
            ScriptToUniv(output.redeem_script, r, false);
            out.pushKV("redeem_script", r);
        }
        if (!output.witness_script.empty()) {
            UniValue r(UniValue::VOBJ);
            ScriptToUniv(output.witness_script, r, false);
            out.pushKV("witness_script", r);
        }
        
        if (!output.hd_keypaths.empty()) {
            UniValue keypaths(UniValue::VARR);
            for (auto entry : output.hd_keypaths) {
                UniValue keypath(UniValue::VOBJ);
                keypath.pushKV("pubkey", HexStr(entry.first));
                keypath.pushKV("master_fingerprint", strprintf("%08x", ReadBE32(entry.second.fingerprint)));
                keypath.pushKV("path", WriteHDKeypath(entry.second.path));