

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.
CScript inner = CreateMultisigRedeemscript(required, pubkeys);
    CTxDestination dest = AddAndGetDestinationForScript(*pwallet, inner, output_type);
    pwallet->SetAddressBook(dest, label, "send");
    UniValue result(UniValue::VOBJ);
    result.pushKV("address", EncodeDestination(dest));
    result.pushKV("redeemScript", HexStr(inner.begin(), inner.end()));
    return result;
}
struct tallyitem
{
    CAmount nAmount;
    int nConf;
    std::vector<uint256> txids;
    bool fIsWatchonly;
    tallyitem()
    {
        nAmount = 0;
        nConf = std::numeric_limits<int>::max();
        fIsWatchonly = false;
    }
};
static UniValue ListReceived(interfaces::Chain::Lock& locked_chain, CWallet * const pwallet, const UniValue& params, bool by_label) EXCLUSIVE_LOCKS_REQUIRED(pwallet->cs_wallet)
{
    LockAnnotation lock(::cs_main); 
    
    int nMinDepth = 1;
    if (!params[0].isNull())
        nMinDepth = params[0].get_int();