

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.
"provided, only that block will be searched and if the transaction is in the mempool or other\n"
                "blocks, or if this node does not have the given block available, the transaction will not be found.\n"
            "DEPRECATED: for now, it also works for transactions with unspent outputs.\n"
            "\nReturn the raw transaction data.\n"
            "\nIf verbose is 'true', returns an Object with information about 'txid'.\n"
            "If verbose is 'false' or omitted, returns a string that is serialized, hex-encoded data for 'txid'.\n"
                ,
                {
                    {"txid", RPCArg::Type::STR_HEX,  false,  "", "The transaction id"},
                    {"verbose", RPCArg::Type::BOOL,  true,  "false", "If false, return a string, otherwise return a json object"},
                    {"blockhash", RPCArg::Type::STR_HEX,  true,  "", "The block in which to look for the transaction"},
                }}
                .ToString() +
            "\nResult (if verbose is not set or set to false):\n"
            "\"data\"      (string) The serialized, hex-encoded data for 'txid'\n"
            "\nResult (if verbose is set to true):\n"
            "{\n"
            "  \"in_active_chain\": b, (bool) Whether specified block is in the active chain or not (only present with explicit \"blockhash\" argument)\n"
            "  \"hex\" : \"data\",       (string) The serialized, hex-encoded data for 'txid'\n"
            "  \"txid\" : \"id\",        (string) The transaction id (same as provided)\n"
            "  \"hash\" : \"id\",        (string) The transaction hash (differs from txid for witness transactions)\n"
            "  \"size\" : n,             (numeric) The serialized transaction size\n"
            "  \"vsize\" : n,            (numeric) The virtual transaction size (differs from size for witness transactions)\n"
            "  \"weight\" : n,           (numeric) The transaction's weight (between vsize*4-3 and vsize*4)\n"
            "  \"version\" : n,          (numeric) The version\n"
            "  \"locktime\" : ttt,       (numeric) The lock time\n"
            "  \"vin\" : [               (array of json objects)\n"
            "     {\n"
            "       \"txid\": \"id\",    (string) The transaction id\n"
            "       \"vout\": n,         (numeric) \n"