

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.
if (g_txindex && !blockindex) {
        f_txindex_ready = g_txindex->BlockUntilSyncedToCurrentChain();
    }
    CTransactionRef tx;
    uint256 hash_block;
    if (!GetTransaction(hash, tx, Params().GetConsensus(), hash_block, true, blockindex)) {
        std::string errmsg;
        if (blockindex) {
            if (!(blockindex->nStatus & BLOCK_HAVE_DATA)) {
                throw JSONRPCError(RPC_MISC_ERROR, "Block not available");
            }
            errmsg = "No such transaction found in the provided block";
        } else if (!g_txindex) {
            errmsg = "No such mempool transaction. Use -txindex to enable blockchain transaction queries";
        } else if (!f_txindex_ready) {
            errmsg = "No such mempool transaction. Blockchain transactions are still in the process of being indexed";
        } else {
            errmsg = "No such mempool or blockchain transaction";
        }
        throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, errmsg + ". Use gettransaction for wallet transactions.");
    }
    if (!fVerbose) {
        return EncodeHexTx(*tx, RPCSerializationFlags());
    }
    UniValue result(UniValue::VOBJ);
    if (blockindex) result.pushKV("in_active_chain", in_active_chain);
    TxToJSON(*tx, hash_block, result);
    return result;
}
static UniValue gettxoutproof(const JSONRPCRequest& request)