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.

{
    return RPCHelpMan{"combinepsbt",
                "\nCombine multiple partially signed Bitcoin transactions into one transaction.\n"
                "Implements the Combiner role.\n",
                {
                    {"txs", RPCArg::Type::ARR, RPCArg::Optional::NO, "The base64 strings of partially signed transactions",
                        {
                            {"psbt", RPCArg::Type::STR, RPCArg::Optional::OMITTED, "A base64 string of a PSBT"},
                        },
                        },
                },
                RPCResult{
                    RPCResult::Type::STR, "", "The base64-encoded partially signed transaction"
                },
                RPCExamples{
                    HelpExampleCli("combinepsbt", R"('["mybase64_1", "mybase64_2", "mybase64_3"]')")