

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.
{
        entry.pushKV("blockhash", wtx.hashBlock.GetHex());
        entry.pushKV("blockindex", wtx.nIndex);
        entry.pushKV("blocktime", LookupBlockIndex(wtx.hashBlock)->GetBlockTime());
    } else {
        entry.pushKV("trusted", wtx.IsTrusted(locked_chain));
    }
    uint256 hash = wtx.GetHash();
    entry.pushKV("txid", hash.GetHex());
    UniValue conflicts(UniValue::VARR);
    for (const uint256& conflict : wtx.GetConflicts())
        conflicts.push_back(conflict.GetHex());
    entry.pushKV("walletconflicts", conflicts);
    entry.pushKV("time", wtx.GetTxTime());
    entry.pushKV("timereceived", (int64_t)wtx.nTimeReceived);
    
    std::string rbfStatus = "no";
    if (confirms <= 0) {
        LOCK(mempool.cs);
        RBFTransactionState rbfState = IsRBFOptIn(*wtx.tx, mempool);
        if (rbfState == RBFTransactionState::UNKNOWN)
            rbfStatus = "unknown";
        else if (rbfState == RBFTransactionState::REPLACEABLE_BIP125)
            rbfStatus = "yes";
    }
    entry.pushKV("bip125-replaceable", rbfStatus);
    for (const std::pair<const std::string, std::string>& item : wtx.mapValue)
        entry.pushKV(item.first, item.second);
}
static std::string LabelFromValue(const UniValue& value)