

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.
BOOST_CHECK(ValueFromAmount(2099999999999999LL).write() == "20999999.99999999");
    BOOST_CHECK_EQUAL(ValueFromAmount(0).write(), "0.00000000");
    BOOST_CHECK_EQUAL(ValueFromAmount((COIN/10000)*123456789).write(), "12345.67890000");
    BOOST_CHECK_EQUAL(ValueFromAmount(-COIN).write(), "-1.00000000");
    BOOST_CHECK_EQUAL(ValueFromAmount(-COIN/10).write(), "-0.10000000");
    BOOST_CHECK_EQUAL(ValueFromAmount(COIN*100000000).write(), "100000000.00000000");
    BOOST_CHECK_EQUAL(ValueFromAmount(COIN*10000000).write(), "10000000.00000000");
    BOOST_CHECK_EQUAL(ValueFromAmount(COIN*1000000).write(), "1000000.00000000");
    BOOST_CHECK_EQUAL(ValueFromAmount(COIN*100000).write(), "100000.00000000");
    BOOST_CHECK_EQUAL(ValueFromAmount(COIN*10000).write(), "10000.00000000");
    BOOST_CHECK_EQUAL(ValueFromAmount(COIN*1000).write(), "1000.00000000");
    BOOST_CHECK_EQUAL(ValueFromAmount(COIN*100).write(), "100.00000000");
    BOOST_CHECK_EQUAL(ValueFromAmount(COIN*10).write(), "10.00000000");
    BOOST_CHECK_EQUAL(ValueFromAmount(COIN).write(), "1.00000000");
    BOOST_CHECK_EQUAL(ValueFromAmount(COIN/10).write(), "0.10000000");
    BOOST_CHECK_EQUAL(ValueFromAmount(COIN/100).write(), "0.01000000");
    BOOST_CHECK_EQUAL(ValueFromAmount(COIN/1000).write(), "0.00100000");
    BOOST_CHECK_EQUAL(ValueFromAmount(COIN/10000).write(), "0.00010000");
    BOOST_CHECK_EQUAL(ValueFromAmount(COIN/100000).write(), "0.00001000");
    BOOST_CHECK_EQUAL(ValueFromAmount(COIN/1000000).write(), "0.00000100");
    BOOST_CHECK_EQUAL(ValueFromAmount(COIN/10000000).write(), "0.00000010");
    BOOST_CHECK_EQUAL(ValueFromAmount(COIN/100000000).write(), "0.00000001");
}
static UniValue ValueFromString(const std::string &str)
{
    UniValue value;
    BOOST_CHECK(value.setNumStr(str));
    return value;
}
BOOST_AUTO_TEST_CASE(rpc_parse_monetary_values)