

Using bitcoin-cli in order to generate a transaction with two op_return generate a wrong transaction.

```
bitcoin-cli createrawtransaction "[]" "{\"data\":\"ee\", \"data\":\"ff\"}"
0200000000020000000000000000036a01ee0000000000000000036a01ee00000000
```

Decoding the hex I obtain the following transaction with two op_return with the same value.

```
bitcoin-cli decoderawtransaction "0200000000020000000000000000036a01ee0000000000000000036a01ee00000000"
{
  "txid": "ed4b691958a573c8569177606c77905ecd07c36eb5eec443f17a45427bb9f5fc",
  "hash": "ed4b691958a573c8569177606c77905ecd07c36eb5eec443f17a45427bb9f5fc",
  "version": 2,
  "size": 34,
  "vsize": 34,
  "weight": 136,
  "locktime": 0,
  "vin": [
  ],
  "vout": [
    {
      "value": 0.00000000,
      "n": 0,
      "scriptPubKey": {
        "asm": "OP_RETURN -110",
        "hex": "6a01ee",
        "type": "nulldata"
      }
    },
    {
      "value": 0.00000000,
      "n": 1,
      "scriptPubKey": {
        "asm": "OP_RETURN -110",
        "hex": "6a01ee",
        "type": "nulldata"
      }
    }
  ]
}
```

This incorrect behavior is related to the duplicate key "data" in the JSON passed to the "createrawtransaction" command.

The problem is related to the use of `outputs[name_].getValStr()` in an JSON with duplicated keys https://github.com/bitcoin/bitcoin/blob/master/src/rpc/rawtransaction.cpp#L432

Although this kind of transaction wouldn't be relayed, I thought about two possible solutions:
* raise an error as it happens for the use of two identical addresses, maybe calling `JSONRPCError(RPC_INVALID_PARAMETER ...` if more than one "data" field is recognized - easier to implement, 
* creation of the transaction correctly containing the two different op_returns, maybe need to pass one array in the data field of the passed JSON (like `"data": ["ee", "ff"]`) - maybe require API change.
}
    
    CDataStream ssTx(SER_NETWORK, PROTOCOL_VERSION);
    ssTx << psbtx;
    return EncodeBase64((unsigned char*)ssTx.data(), ssTx.size());
}
UniValue converttopsbt(const JSONRPCRequest& request)
{
    if (request.fHelp || request.params.size() < 1 || request.params.size() > 3)
        throw std::runtime_error(
            RPCHelpMan{"converttopsbt",
                "\nConverts a network serialized transaction to a PSBT. This should be used only with createrawtransaction and fundrawtransaction\n"
                "createpsbt and walletcreatefundedpsbt should be used for new applications.\n",
                {
                    {"hexstring", RPCArg::Type::STR_HEX,  false,  "", "The hex string of a raw transaction"},
                    {"permitsigdata", RPCArg::Type::BOOL,  true,  "false", "If true, any signatures in the input will be discarded and conversion.\n"
                            "                              will continue. If false, RPC will fail if any signatures are present."},
                    {"iswitness", RPCArg::Type::BOOL,  true,  "", "Whether the transaction hex is a serialized witness transaction.\n"
                            "                              If iswitness is not present, heuristic tests will be used in decoding. If true, only witness deserializaion\n"
                            "                              will be tried. If false, only non-witness deserialization will be tried. Only has an effect if\n"
                            "                              permitsigdata is true."},
                }}
                .ToString() +
                            "\nResult:\n"
                            "  \"psbt\"        (string)  The resulting raw transaction (base64-encoded string)\n"
                            "\nExamples:\n"
                            "\nCreate a transaction\n"
                            + HelpExampleCli("createrawtransaction", "\"[{\\\"txid\\\":\\\"myid\\\",\\\"vout\\\":0}]\" \"[{\\\"data\\\":\\\"00010203\\\"}]\"") +
                            "\nConvert the transaction to a PSBT\n"
                            + HelpExampleCli("converttopsbt", "\"rawtransaction\"")