

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.
CAmount result3[NUM_GETBLOCKSTATS_PERCENTILES] = { 0 };
    feerates.clear();
    feerates.emplace_back(std::make_pair(1, 9));
    feerates.emplace_back(std::make_pair(2 , 11)); 
    feerates.emplace_back(std::make_pair(2 , 5)); 
    feerates.emplace_back(std::make_pair(4 ,50)); 
    feerates.emplace_back(std::make_pair(5 ,10));
    feerates.emplace_back(std::make_pair(9 ,15)); 
    CalculatePercentilesByWeight(result3, feerates, total_weight);
    BOOST_CHECK_EQUAL(result3[0], 2);
    BOOST_CHECK_EQUAL(result3[1], 2);
    BOOST_CHECK_EQUAL(result3[2], 4);
    BOOST_CHECK_EQUAL(result3[3], 4);
    BOOST_CHECK_EQUAL(result3[4], 9);
    
    total_weight = 104;
    CAmount result4[NUM_GETBLOCKSTATS_PERCENTILES] = { 0 };
    feerates.clear();
    feerates.emplace_back(std::make_pair(1, 100));
    feerates.emplace_back(std::make_pair(2, 1));
    feerates.emplace_back(std::make_pair(3, 1));
    feerates.emplace_back(std::make_pair(3, 1));
    feerates.emplace_back(std::make_pair(999999, 1));
    CalculatePercentilesByWeight(result4, feerates, total_weight);
    for (int64_t i = 0; i < NUM_GETBLOCKSTATS_PERCENTILES; i++) {
        BOOST_CHECK_EQUAL(result4[i], 1);
    }
}
BOOST_AUTO_TEST_SUITE_END()