

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.
+ HelpExampleCli("listtransactions", "\"*\" 20 100") +
            "\nAs a JSON-RPC call\n"
            + HelpExampleRpc("listtransactions", "\"*\", 20, 100")
        );
    
    
    pwallet->BlockUntilSyncedToCurrentChain();
    const std::string* filter_label = nullptr;
    if (!request.params[0].isNull() && request.params[0].get_str() != "*") {
        filter_label = &request.params[0].get_str();
        if (filter_label->empty()) {
            throw JSONRPCError(RPC_INVALID_PARAMETER, "Label argument must be a valid label name or \"*\".");
        }
    }
    int nCount = 10;
    if (!request.params[1].isNull())
        nCount = request.params[1].get_int();
    int nFrom = 0;
    if (!request.params[2].isNull())
        nFrom = request.params[2].get_int();
    isminefilter filter = ISMINE_SPENDABLE;
    if(!request.params[3].isNull())
        if(request.params[3].get_bool())
            filter = filter | ISMINE_WATCH_ONLY;
    if (nCount < 0)
        throw JSONRPCError(RPC_INVALID_PARAMETER, "Negative count");
    if (nFrom < 0)
        throw JSONRPCError(RPC_INVALID_PARAMETER, "Negative from");
    UniValue ret(UniValue::VARR);
    {