

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.
UniValueType(), 
        UniValue::VNUM,
        UniValue::VBOOL
        }, true
    );
    CMutableTransaction rawTx = ConstructTransaction(request.params[0], request.params[1], request.params[2], request.params[3]);
    return EncodeHexTx(rawTx);
}
static UniValue decoderawtransaction(const JSONRPCRequest& request)
{
    if (request.fHelp || request.params.size() < 1 || request.params.size() > 2)
        throw std::runtime_error(
            RPCHelpMan{"decoderawtransaction",
                "\nReturn a JSON object representing the serialized, hex-encoded transaction.\n",
                {
                    {"hexstring", RPCArg::Type::STR_HEX,  false,  "", "The transaction hex string"},
                    {"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"},
                }}
                .ToString() +
            "\nResult:\n"
            "{\n"
            "  \"txid\" : \"id\",        (string) The transaction id\n"
            "  \"hash\" : \"id\",        (string) The transaction hash (differs from txid for witness transactions)\n"
            "  \"size\" : n,             (numeric) The transaction size\n"
            "  \"vsize\" : n,            (numeric) The virtual transaction size (differs from size for witness transactions)\n"
            "  \"weight\" : n,           (numeric) The transaction's weight (between vsize*4 - 3 and vsize*4)\n"
            "  \"version\" : n,          (numeric) The version\n"
            "  \"locktime\" : ttt,       (numeric) The lock time\n"
            "  \"vin\" : [               (array of json objects)\n"