

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.
"}\n"
            "\nExamples:\n"
            + HelpExampleCli("getrawtransaction", "\"mytxid\"")
            + HelpExampleCli("getrawtransaction", "\"mytxid\" true")
            + HelpExampleRpc("getrawtransaction", "\"mytxid\", true")
            + HelpExampleCli("getrawtransaction", "\"mytxid\" false \"myblockhash\"")
            + HelpExampleCli("getrawtransaction", "\"mytxid\" true \"myblockhash\"")
        );
    bool in_active_chain = true;
    uint256 hash = ParseHashV(request.params[0], "parameter 1");
    CBlockIndex* blockindex = nullptr;
    if (hash == Params().GenesisBlock().hashMerkleRoot) {
        
        throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "The genesis block coinbase is not considered an ordinary transaction and cannot be retrieved");
    }
    
    bool fVerbose = false;
    if (!request.params[1].isNull()) {
        fVerbose = request.params[1].isNum() ? (request.params[1].get_int() != 0) : request.params[1].get_bool();
    }
    if (!request.params[2].isNull()) {
        LOCK(cs_main);
        uint256 blockhash = ParseHashV(request.params[2], "parameter 3");
        blockindex = LookupBlockIndex(blockhash);
        if (!blockindex) {
            throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "Block hash not found");
        }
        in_active_chain = chainActive.Contains(blockindex);
    }
    bool f_txindex_ready = false;