

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.
LOCK(pwallet->cs_wallet);
    UniValue ret(UniValue::VOBJ);
    CTxDestination dest = DecodeDestination(request.params[0].get_str());
    
    if (!IsValidDestination(dest)) {
        throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "Invalid address");
    }
    std::string currentAddress = EncodeDestination(dest);
    ret.pushKV("address", currentAddress);
    CScript scriptPubKey = GetScriptForDestination(dest);
    ret.pushKV("scriptPubKey", HexStr(scriptPubKey.begin(), scriptPubKey.end()));
    isminetype mine = IsMine(*pwallet, dest);
    ret.pushKV("ismine", bool(mine & ISMINE_SPENDABLE));
    bool solvable = IsSolvable(*pwallet, scriptPubKey);
    ret.pushKV("solvable", solvable);
    if (solvable) {
       ret.pushKV("desc", InferDescriptor(scriptPubKey, *pwallet)->ToString());
    }
    ret.pushKV("iswatchonly", bool(mine & ISMINE_WATCH_ONLY));
    ret.pushKV("solvable", IsSolvable(*pwallet, scriptPubKey));
    UniValue detail = DescribeWalletAddress(pwallet, dest);
    ret.pushKVs(detail);
    if (pwallet->mapAddressBook.count(dest)) {
        ret.pushKV("label", pwallet->mapAddressBook[dest].name);
    }
    ret.pushKV("ischange", pwallet->IsChange(scriptPubKey));
    const CKeyMetadata* meta = nullptr;
    CKeyID key_id = GetKeyForDestination(*pwallet, dest);
    if (!key_id.IsNull()) {
        auto it = pwallet->mapKeyMetadata.find(key_id);