

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.
const uint256& tx_hash = tx->GetHash();
    CAmount max_raw_tx_fee = ::maxTxFee;
    if (!request.params[1].isNull() && request.params[1].get_bool()) {
        max_raw_tx_fee = 0;
    }
    UniValue result(UniValue::VARR);
    UniValue result_0(UniValue::VOBJ);
    result_0.pushKV("txid", tx_hash.GetHex());
    CValidationState state;
    bool missing_inputs;
    bool test_accept_res;
    {
        LOCK(cs_main);
        test_accept_res = AcceptToMemoryPool(mempool, state, std::move(tx), &missing_inputs,
            nullptr , false , max_raw_tx_fee,  true);
    }
    result_0.pushKV("allowed", test_accept_res);
    if (!test_accept_res) {
        if (state.IsInvalid()) {
            result_0.pushKV("reject-reason", strprintf("%i: %s", state.GetRejectCode(), state.GetRejectReason()));
        } else if (missing_inputs) {
            result_0.pushKV("reject-reason", "missing-inputs");
        } else {
            result_0.pushKV("reject-reason", state.GetRejectReason());
        }
    }
    result.push_back(std::move(result_0));
    return result;
}
static std::string WriteHDKeypath(std::vector<uint32_t>& keypath)