

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.
{
    UniValue r;
    
    std::string prevout =
      "[{\"txid\":\"b4cc287e58f87cdae59417329f710f3ecd75a4ee1d2872b7248f50977c8493f3\","
      "\"vout\":1,\"scriptPubKey\":\"a914b10c9df5f7edf436c697f02f1efdba4cf399615187\","
      "\"redeemScript\":\"512103debedc17b3df2badbcdd86d5feb4562b86fe182e5998abd8bcd4f122c6155b1b21027e940bb73ab8732bfdf7f9216ecefca5b94d6df834e77e108f68e66f126044c052ae\"}]";
    r = CallRPC(std::string("createrawtransaction ")+prevout+" "+
      "{\"3HqAe9LtNBjnsfM4CyYaWTnvCaUYT7v4oZ\":11}");
    std::string notsigned = r.get_str();
    std::string privkey1 = "\"KzsXybp9jX64P5ekX1KUxRQ79Jht9uzW7LorgwE65i5rWACL6LQe\"";
    std::string privkey2 = "\"Kyhdf5LuKTRx4ge69ybABsiUAWjVRK4XGxAKk2FQLp2HjGMy87Z4\"";
    InitInterfaces interfaces;
    interfaces.chain = interfaces::MakeChain();
    g_rpc_interfaces = &interfaces;
    r = CallRPC(std::string("signrawtransactionwithkey ")+notsigned+" [] "+prevout);
    BOOST_CHECK(find_value(r.get_obj(), "complete").get_bool() == false);
    r = CallRPC(std::string("signrawtransactionwithkey ")+notsigned+" ["+privkey1+","+privkey2+"] "+prevout);
    BOOST_CHECK(find_value(r.get_obj(), "complete").get_bool() == true);
    g_rpc_interfaces = nullptr;
}
BOOST_AUTO_TEST_CASE(rpc_createraw_op_return)