

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.
int64_t now = GetTime();
    BOOST_CHECK(banned_until.get_int64() > now);
    BOOST_CHECK(banned_until.get_int64()-now <= 200);
    
    BOOST_CHECK_THROW(r = CallRPC(std::string("setban 127.0.0.1 add")), std::runtime_error);
    BOOST_CHECK_NO_THROW(CallRPC(std::string("setban 127.0.0.0/24 remove")));
    BOOST_CHECK_NO_THROW(r = CallRPC(std::string("listbanned")));
    ar = r.get_array();
    BOOST_CHECK_EQUAL(ar.size(), 0U);
    BOOST_CHECK_NO_THROW(r = CallRPC(std::string("setban 127.0.0.0/255.255.0.0 add")));
    BOOST_CHECK_THROW(r = CallRPC(std::string("setban 127.0.1.1 add")), std::runtime_error);
    BOOST_CHECK_NO_THROW(CallRPC(std::string("clearbanned")));
    BOOST_CHECK_NO_THROW(r = CallRPC(std::string("listbanned")));
    ar = r.get_array();
    BOOST_CHECK_EQUAL(ar.size(), 0U);
    BOOST_CHECK_THROW(r = CallRPC(std::string("setban test add")), std::runtime_error); 
    
    BOOST_CHECK_NO_THROW(r = CallRPC(std::string("setban FE80:0000:0000:0000:0202:B3FF:FE1E:8329 add")));
    BOOST_CHECK_NO_THROW(r = CallRPC(std::string("listbanned")));
    ar = r.get_array();
    o1 = ar[0].get_obj();
    adr = find_value(o1, "address");
    BOOST_CHECK_EQUAL(adr.get_str(), "fe80::202:b3ff:fe1e:8329/128");
    BOOST_CHECK_NO_THROW(CallRPC(std::string("clearbanned")));
    BOOST_CHECK_NO_THROW(r = CallRPC(std::string("setban 2001:db8::/ffff:fffc:0:0:0:0:0:0 add")));
    BOOST_CHECK_NO_THROW(r = CallRPC(std::string("listbanned")));
    ar = r.get_array();
    o1 = ar[0].get_obj();
    adr = find_value(o1, "address");
    BOOST_CHECK_EQUAL(adr.get_str(), "2001:db8::/30");