

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.
.ToString() +
            "\nResult:\n"
            "[                   (array) The result of the mempool acceptance test for each raw transaction in the input array.\n"
            "                            Length is exactly one for now.\n"
            " {\n"
            "  \"txid\"           (string) The transaction hash in hex\n"
            "  \"allowed\"        (boolean) If the mempool allows this tx to be inserted\n"
            "  \"reject-reason\"  (string) Rejection string (only present when 'allowed' is false)\n"
            " }\n"
            "]\n"
            "\nExamples:\n"
            "\nCreate a transaction\n"
            + HelpExampleCli("createrawtransaction", "\"[{\\\"txid\\\" : \\\"mytxid\\\",\\\"vout\\\":0}]\" \"{\\\"myaddress\\\":0.01}\"") +
            "Sign the transaction, and get back the hex\n"
            + HelpExampleCli("signrawtransactionwithwallet", "\"myhex\"") +
            "\nTest acceptance of the transaction (signed hex)\n"
            + HelpExampleCli("testmempoolaccept", "\"signedhex\"") +
            "\nAs a JSON-RPC call\n"
            + HelpExampleRpc("testmempoolaccept", "[\"signedhex\"]")
            );
    }
    RPCTypeCheck(request.params, {UniValue::VARR, UniValue::VBOOL});
    if (request.params[0].get_array().size() != 1) {
        throw JSONRPCError(RPC_INVALID_PARAMETER, "Array must contain exactly one raw transaction for now");
    }
    CMutableTransaction mtx;
    if (!DecodeHexTx(mtx, request.params[0].get_array()[0].get_str())) {
        throw JSONRPCError(RPC_DESERIALIZATION_ERROR, "TX decode failed");
    }
    CTransactionRef tx(MakeTransactionRef(std::move(mtx)));