

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.
"and throwing an RPC error if the block is not in our best chain\n",
                {
                    {"proof", RPCArg::Type::STR_HEX,  false,  "", "The hex-encoded proof generated by gettxoutproof"},
                }}
                .ToString() +
            "\nResult:\n"
            "[\"txid\"]      (array, strings) The txid(s) which the proof commits to, or empty array if the proof can not be validated.\n"
        );
    CDataStream ssMB(ParseHexV(request.params[0], "proof"), SER_NETWORK, PROTOCOL_VERSION | SERIALIZE_TRANSACTION_NO_WITNESS);
    CMerkleBlock merkleBlock;
    ssMB >> merkleBlock;
    UniValue res(UniValue::VARR);
    std::vector<uint256> vMatch;
    std::vector<unsigned int> vIndex;
    if (merkleBlock.txn.ExtractMatches(vMatch, vIndex) != merkleBlock.header.hashMerkleRoot)
        return res;
    LOCK(cs_main);
    const CBlockIndex* pindex = LookupBlockIndex(merkleBlock.header.GetHash());
    if (!pindex || !chainActive.Contains(pindex) || pindex->nTx == 0) {
        throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "Block not found in chain");
    }
    
    if (pindex->nTx == merkleBlock.txn.GetNumTransactions()) {
        for (const uint256& hash : vMatch) {
            res.push_back(hash.GetHex());
        }
    }
    return res;
}
CMutableTransaction ConstructTransaction(const UniValue& inputs_in, const UniValue& outputs_in, const UniValue& locktime, const UniValue& rbf)