

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.
nSequence = MAX_BIP125_RBF_SEQUENCE; 
        } else if (rawTx.nLockTime) {
            nSequence = CTxIn::SEQUENCE_FINAL - 1;
        } else {
            nSequence = CTxIn::SEQUENCE_FINAL;
        }
        
        const UniValue& sequenceObj = find_value(o, "sequence");
        if (sequenceObj.isNum()) {
            int64_t seqNr64 = sequenceObj.get_int64();
            if (seqNr64 < 0 || seqNr64 > CTxIn::SEQUENCE_FINAL) {
                throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid parameter, sequence number is out of range");
            } else {
                nSequence = (uint32_t)seqNr64;
            }
        }
        CTxIn in(COutPoint(txid, nOutput), CScript(), nSequence);
        rawTx.vin.push_back(in);
    }
    std::set<CTxDestination> destinations;
    if (!outputs_is_obj) {
        
        UniValue outputs_dict = UniValue(UniValue::VOBJ);
        for (size_t i = 0; i < outputs.size(); ++i) {
            const UniValue& output = outputs[i];
            if (!output.isObject()) {
                throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid parameter, key-value pair not an object as expected");
            }
            if (output.size() != 1) {
                throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid parameter, key-value pair must contain exactly one key");