

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.
}
            outputs_dict.pushKVs(output);
        }
        outputs = std::move(outputs_dict);
    }
    for (const std::string& name_ : outputs.getKeys()) {
        if (name_ == "data") {
            std::vector<unsigned char> data = ParseHexV(outputs[name_].getValStr(), "Data");
            CTxOut out(0, CScript() << OP_RETURN << data);
            rawTx.vout.push_back(out);
        } else {
            CTxDestination destination = DecodeDestination(name_);
            if (!IsValidDestination(destination)) {
                throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, std::string("Invalid Bitcoin address: ") + name_);
            }
            if (!destinations.insert(destination).second) {
                throw JSONRPCError(RPC_INVALID_PARAMETER, std::string("Invalid parameter, duplicated address: ") + name_);
            }
            CScript scriptPubKey = GetScriptForDestination(destination);
            CAmount nAmount = AmountFromValue(outputs[name_]);
            CTxOut out(nAmount, scriptPubKey);
            rawTx.vout.push_back(out);
        }
    }
    if (!rbf.isNull() && rawTx.vin.size() > 0 && rbfOptIn != SignalsOptInRBF(rawTx)) {
        throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid parameter combination: Sequence number(s) contradict replaceable option");
    }
    return rawTx;
}
static UniValue createrawtransaction(const JSONRPCRequest& request)