

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.
"  \"reqSigs\": n,    (numeric) The required signatures\n"
            "  \"addresses\": [   (json array of string)\n"
            "     \"address\"     (string) bitcoin address\n"
            "     ,...\n"
            "  ],\n"
            "  \"p2sh\",\"address\" (string) address of P2SH script wrapping this redeem script (not returned if the script is already a P2SH).\n"
            "}\n"
            "\nExamples:\n"
            + HelpExampleCli("decodescript", "\"hexstring\"")
            + HelpExampleRpc("decodescript", "\"hexstring\"")
        );
    RPCTypeCheck(request.params, {UniValue::VSTR});
    UniValue r(UniValue::VOBJ);
    CScript script;
    if (request.params[0].get_str().size() > 0){
        std::vector<unsigned char> scriptData(ParseHexV(request.params[0], "argument"));
        script = CScript(scriptData.begin(), scriptData.end());
    } else {
        
    }
    ScriptPubKeyToUniv(script, r, false);
    UniValue type;
    type = find_value(r, "type");
    if (type.isStr() && type.get_str() != "scripthash") {
        
        
        r.pushKV("p2sh", EncodeDestination(CScriptID(script)));
        
        
        if (type.get_str() == "pubkey" || type.get_str() == "pubkeyhash" || type.get_str() == "multisig" || type.get_str() == "nonstandard") {