

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.
"          \"type\" : \"pubkeyhash\",    (string) The type, eg 'pubkeyhash'\n"
            "          \"address\" : \"address\"     (string) Bitcoin address if there is one\n"
            "        }\n"
            "      },\n"
            "      \"partial_signatures\" : {             (json object, optional)\n"
            "        \"pubkey\" : \"signature\",           (string) The public key and signature that corresponds to it.\n"
            "        ,...\n"
            "      }\n"
            "      \"sighash\" : \"type\",                  (string, optional) The sighash type to be used\n"
            "      \"redeem_script\" : {       (json object, optional)\n"
            "          \"asm\" : \"asm\",            (string) The asm\n"
            "          \"hex\" : \"hex\",            (string) The hex\n"
            "          \"type\" : \"pubkeyhash\",    (string) The type, eg 'pubkeyhash'\n"
            "        }\n"
            "      \"witness_script\" : {       (json object, optional)\n"
            "          \"asm\" : \"asm\",            (string) The asm\n"
            "          \"hex\" : \"hex\",            (string) The hex\n"
            "          \"type\" : \"pubkeyhash\",    (string) The type, eg 'pubkeyhash'\n"
            "        }\n"
            "      \"bip32_derivs\" : {          (json object, optional)\n"
            "        \"pubkey\" : {                     (json object, optional) The public key with the derivation path as the value.\n"
            "          \"master_fingerprint\" : \"fingerprint\"     (string) The fingerprint of the master key\n"
            "          \"path\" : \"path\",                         (string) The path\n"
            "        }\n"
            "        ,...\n"
            "      }\n"
            "      \"final_scriptsig\" : {       (json object, optional)\n"
            "          \"asm\" : \"asm\",            (string) The asm\n"
            "          \"hex\" : \"hex\",            (string) The hex\n"
            "        }\n"