

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.
if (has_filtered_address && !(filtered_address == address)) {
                continue;
            }
            isminefilter mine = IsMine(*pwallet, address);
            if(!(mine & filter))
                continue;
            tallyitem& item = mapTally[address];
            item.nAmount += txout.nValue;
            item.nConf = std::min(item.nConf, nDepth);
            item.txids.push_back(wtx.GetHash());
            if (mine & ISMINE_WATCH_ONLY)
                item.fIsWatchonly = true;
        }
    }
    
    UniValue ret(UniValue::VARR);
    std::map<std::string, tallyitem> label_tally;
    
    
    auto start = pwallet->mapAddressBook.begin();
    auto end = pwallet->mapAddressBook.end();
    
    if (has_filtered_address) {
        start = pwallet->mapAddressBook.find(filtered_address);
        if (start != end) {
            end = std::next(start);
        }
    }
    for (auto item_it = start; item_it != end; ++item_it)
    {