

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.
"Optionally filter to only include txouts paid to specified addresses.\n",
                {
                    {"minconf", RPCArg::Type::NUM,  true,  "1", "The minimum confirmations to filter"},
                    {"maxconf", RPCArg::Type::NUM,  true,  "9999999", "The maximum confirmations to filter"},
                    {"addresses", RPCArg::Type::ARR,  true,  "", "A json array of bitcoin addresses to filter",
                        {
                            {"address", RPCArg::Type::STR,  true,  "", "bitcoin address"},
                        },
                    },
                    {"include_unsafe", RPCArg::Type::BOOL,  true,  "true", "Include outputs that are not safe to spend\n"
            "                  See description of \"safe\" attribute below."},
                    {"query_options", RPCArg::Type::OBJ,  true,  "", "JSON with query options",
                        {
                            {"minimumAmount", RPCArg::Type::AMOUNT,  true,  "0", "Minimum value of each UTXO in " + CURRENCY_UNIT + ""},
                            {"maximumAmount", RPCArg::Type::AMOUNT,  true,  "unlimited", "Maximum value of each UTXO in " + CURRENCY_UNIT + ""},
                            {"maximumCount", RPCArg::Type::NUM,  true,  "unlimited", "Maximum number of UTXOs"},
                            {"minimumSumAmount", RPCArg::Type::AMOUNT,  true,  "unlimited", "Minimum sum value of all UTXOs in " + CURRENCY_UNIT + ""},
                        },
                        "query_options"},
                }}
                .ToString() +
            "\nResult\n"
            "[                   (array of json object)\n"
            "  {\n"
            "    \"txid\" : \"txid\",          (string) the transaction id \n"
            "    \"vout\" : n,               (numeric) the vout value\n"
            "    \"address\" : \"address\",    (string) the bitcoin address\n"
            "    \"label\" : \"label\",        (string) The associated label, or \"\" for the default label\n"
            "    \"scriptPubKey\" : \"key\",   (string) the script key\n"
            "    \"amount\" : x.xxx,         (numeric) the transaction output amount in " + CURRENCY_UNIT + "\n"