

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.
"  \"witness_program\" : \"hex\"     (string, optional) The hex value of the witness program\n"
            "  \"script\" : \"type\"             (string, optional) The output script type. Only if \"isscript\" is true and the redeemscript is known. Possible types: nonstandard, pubkey, pubkeyhash, scripthash, multisig, nulldata, witness_v0_keyhash, witness_v0_scripthash, witness_unknown\n"
            "  \"hex\" : \"hex\",                (string, optional) The redeemscript for the p2sh address\n"
            "  \"pubkeys\"                     (string, optional) Array of pubkeys associated with the known redeemscript (only if \"script\" is \"multisig\")\n"
            "    [\n"
            "      \"pubkey\"\n"
            "      ,...\n"
            "    ]\n"
            "  \"sigsrequired\" : xxxxx        (numeric, optional) Number of signatures required to spend multisig output (only if \"script\" is \"multisig\")\n"
            "  \"pubkey\" : \"publickeyhex\",    (string, optional) The hex value of the raw public key, for single-key addresses (possibly embedded in P2SH or P2WSH)\n"
            "  \"embedded\" : {...},           (object, optional) Information about the address embedded in P2SH or P2WSH, if relevant and known. It includes all getaddressinfo output fields for the embedded address, excluding metadata (\"timestamp\", \"hdkeypath\", \"hdseedid\") and relation to the wallet (\"ismine\", \"iswatchonly\").\n"
            "  \"iscompressed\" : true|false,  (boolean) If the address is compressed\n"
            "  \"label\" :  \"label\"         (string) The label associated with the address, \"\" is the default label\n"
            "  \"timestamp\" : timestamp,      (number, optional) The creation time of the key if available in seconds since epoch (Jan 1 1970 GMT)\n"
            "  \"hdkeypath\" : \"keypath\"       (string, optional) The HD keypath if the key is HD and available\n"
            "  \"hdseedid\" : \"<hash160>\"      (string, optional) The Hash160 of the HD seed\n"
            "  \"hdmasterkeyid\" : \"<hash160>\" (string, optional) alias for hdseedid maintained for backwards compatibility. Will be removed in V0.18.\n"
            "  \"labels\"                      (object) Array of labels associated with the address.\n"
            "    [\n"
            "      { (json object of label data)\n"
            "        \"name\": \"labelname\" (string) The label\n"
            "        \"purpose\": \"string\" (string) Purpose of address (\"send\" for sending address, \"receive\" for receiving address)\n"
            "      },...\n"
            "    ]\n"
            "}\n"
            "\nExamples:\n"
            + HelpExampleCli("getaddressinfo", "\"1PSSGeFHDnKNxiEyFrD1wcEaHr9hrQDDWc\"")
            + HelpExampleRpc("getaddressinfo", "\"1PSSGeFHDnKNxiEyFrD1wcEaHr9hrQDDWc\"")
        );
    }