

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.
"[\n"
            "  {\n"
            "    \"address\":\"address\",    (string) The bitcoin address of the transaction.\n"
            "    \"category\":\"send|receive\", (string) The transaction category.\n"
            "    \"amount\": x.xxx,          (numeric) The amount in " + CURRENCY_UNIT + ". This is negative for the 'send' category, and is positive\n"
            "                                        for the 'receive' category,\n"
            "    \"label\": \"label\",       (string) A comment for the address/transaction, if any\n"
            "    \"vout\": n,                (numeric) the vout value\n"
            "    \"fee\": x.xxx,             (numeric) The amount of the fee in " + CURRENCY_UNIT + ". This is negative and only available for the \n"
            "                                         'send' category of transactions.\n"
            "    \"confirmations\": n,       (numeric) The number of confirmations for the transaction. Negative confirmations indicate the\n"
            "                                         transaction conflicts with the block chain\n"
            "    \"trusted\": xxx,           (bool) Whether we consider the outputs of this unconfirmed transaction safe to spend.\n"
            "    \"blockhash\": \"hashvalue\", (string) The block hash containing the transaction.\n"
            "    \"blockindex\": n,          (numeric) The index of the transaction in the block that includes it.\n"
            "    \"blocktime\": xxx,         (numeric) The block time in seconds since epoch (1 Jan 1970 GMT).\n"
            "    \"txid\": \"transactionid\", (string) The transaction id.\n"
            "    \"time\": xxx,              (numeric) The transaction time in seconds since epoch (midnight Jan 1 1970 GMT).\n"
            "    \"timereceived\": xxx,      (numeric) The time received in seconds since epoch (midnight Jan 1 1970 GMT).\n"
            "    \"comment\": \"...\",       (string) If a comment is associated with the transaction.\n"
            "    \"bip125-replaceable\": \"yes|no|unknown\",  (string) Whether this transaction could be replaced due to BIP125 (replace-by-fee);\n"
            "                                                     may be unknown for unconfirmed transactions not in the mempool\n"
            "    \"abandoned\": xxx          (bool) 'true' if the transaction has been abandoned (inputs are respendable). Only available for the \n"
            "                                         'send' category of transactions.\n"
            "  }\n"
            "]\n"
            "\nExamples:\n"
            "\nList the most recent 10 transactions in the systems\n"
            + HelpExampleCli("listtransactions", "") +
            "\nList transactions 100 to 120\n"