

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.
"\nList received transactions by label.\n",
                {
                    {"minconf", RPCArg::Type::NUM,  true,  "1", "The minimum number of confirmations before payments are included."},
                    {"include_empty", RPCArg::Type::BOOL,  true,  "false", "Whether to include labels that haven't received any payments."},
                    {"include_watchonly", RPCArg::Type::BOOL,  true,  "false", "Whether to include watch-only addresses (see 'importaddress')."},
                }}
                .ToString() +
            "\nResult:\n"
            "[\n"
            "  {\n"
            "    \"involvesWatchonly\" : true,   (bool) Only returned if imported addresses were involved in transaction\n"
            "    \"amount\" : x.xxx,             (numeric) The total amount received by addresses with this label\n"
            "    \"confirmations\" : n,          (numeric) The number of confirmations of the most recent transaction included\n"
            "    \"label\" : \"label\"           (string) The label of the receiving address. The default label is \"\".\n"
            "  }\n"
            "  ,...\n"
            "]\n"
            "\nExamples:\n"
            + HelpExampleCli("listreceivedbylabel", "")
            + HelpExampleCli("listreceivedbylabel", "6 true")
            + HelpExampleRpc("listreceivedbylabel", "6, true, true")
        );
    
    
    pwallet->BlockUntilSyncedToCurrentChain();
    auto locked_chain = pwallet->chain().lock();
    LOCK(pwallet->cs_wallet);
    return ListReceived(*locked_chain, pwallet, request.params, true);
}
static void MaybePushAddress(UniValue & entry, const CTxDestination &dest)