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{"createrawtransaction",
                "\nCreate a transaction spending the given inputs and creating new outputs.\n"
                "Outputs can be addresses or data.\n"
                "Returns hex-encoded raw transaction.\n"
                "Note that the transaction's inputs are not signed, and\n"
                "it is not stored in the wallet or transmitted to the network.\n",
                {
                    {"inputs", RPCArg::Type::ARR, RPCArg::Optional::NO, "The inputs",
                        {
                            {"", RPCArg::Type::OBJ, RPCArg::Optional::OMITTED, "",
                                {
                                    {"txid", RPCArg::Type::STR_HEX, RPCArg::Optional::NO, "The transaction id"},
                                    {"vout", RPCArg::Type::NUM, RPCArg::Optional::NO, "The output number"},
                                    {"sequence", RPCArg::Type::NUM,  "depends on the value of the 'replaceable' and 'locktime' arguments", "The sequence number"},
                                },
                                },
                        },
                        },
                    {"outputs", RPCArg::Type::ARR, RPCArg::Optional::NO, "The outputs (key-value pairs), where none of the keys are duplicated.\n"
                            "That is, each address can only appear once and there can only be one 'data' object.\n"
                            "For compatibility reasons, a dictionary, which holds the key-value pairs directly, is also\n"
                            "                             accepted as second parameter.",
                        {
                            {"", RPCArg::Type::OBJ, RPCArg::Optional::OMITTED, "",
                                {
                                    {"address", RPCArg::Type::AMOUNT, RPCArg::Optional::NO, "A key-value pair. The key (string) is the bitcoin address, the value (float or string) is the amount in " + CURRENCY_UNIT},
                                },
                                },
                            {"", RPCArg::Type::OBJ, RPCArg::Optional::OMITTED, "",