

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.
{
        CTransactionRef tx;
        if (!GetTransaction(oneTxid, tx, Params().GetConsensus(), hashBlock, false) || hashBlock.IsNull())
            throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "Transaction not yet in block");
        pblockindex = LookupBlockIndex(hashBlock);
        if (!pblockindex) {
            throw JSONRPCError(RPC_INTERNAL_ERROR, "Transaction index corrupt");
        }
    }
    CBlock block;
    if(!ReadBlockFromDisk(block, pblockindex, Params().GetConsensus()))
        throw JSONRPCError(RPC_INTERNAL_ERROR, "Can't read block from disk");
    unsigned int ntxFound = 0;
    for (const auto& tx : block.vtx)
        if (setTxids.count(tx->GetHash()))
            ntxFound++;
    if (ntxFound != setTxids.size())
        throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "Not all transactions found in specified or retrieved block");
    CDataStream ssMB(SER_NETWORK, PROTOCOL_VERSION | SERIALIZE_TRANSACTION_NO_WITNESS);
    CMerkleBlock mb(block, setTxids);
    ssMB << mb;
    std::string strHex = HexStr(ssMB.begin(), ssMB.end());
    return strHex;
}
static UniValue verifytxoutproof(const JSONRPCRequest& request)
{
    if (request.fHelp || request.params.size() != 1)
        throw std::runtime_error(
            RPCHelpMan{"verifytxoutproof",
                "\nVerifies that a proof points to a transaction in a block, returning the transaction it commits to\n"