On master, reported on IRC, appveyor and travis (macos):

Example backtrace:

```
wallet_multiwallet.py --usecli failed, Duration: 56 s
stdout:
2018-12-10T19:52:57.293000Z TestFramework (INFO): Initializing test directory C:\Users\appveyor\AppData\Local\Temp\1\test_runner_20181210_194416\wallet_multiwallet_70
2018-12-10T19:53:18.569000Z TestFramework (INFO): Do not allow -zapwallettxes with multiwallet
2018-12-10T19:53:19.332000Z TestFramework (INFO): Do not allow -salvagewallet with multiwallet
2018-12-10T19:53:19.839000Z TestFramework (INFO): Do not allow -upgradewallet with multiwallet
2018-12-10T19:53:32.857000Z TestFramework (INFO): Check for per-wallet settxfee call
2018-12-10T19:53:33.296000Z TestFramework (INFO): Test dynamic wallet loading
2018-12-10T19:53:36.907000Z TestFramework (INFO): Load first wallet
2018-12-10T19:53:37.476000Z TestFramework (INFO): Load second wallet
2018-12-10T19:53:38.049000Z TestFramework (INFO): Load remaining wallets
2018-12-10T19:53:41.485000Z TestFramework (INFO): Test dynamic wallet creation.
2018-12-10T19:53:43.012000Z TestFramework (INFO): Test dynamic wallet unloading
2018-12-10T19:53:47.773000Z TestFramework (INFO): Test wallet backup
2018-12-10T19:53:52.517000Z TestFramework (ERROR): JSONRPC error
Traceback (most recent call last):
  File "C:\projects\bitcoin\test\functional\test_framework\test_framework.py", line 173, in main
    self.run_test()
  File "C:\projects\bitcoin/test/functional/wallet_multiwallet.py", line 315, in run_test
    self.nodes[0].loadwallet(wallet_name)
  File "C:\projects\bitcoin\test\functional\test_framework\test_node.py", line 400, in __call__
    return self.cli.send_cli(self.command, *args, **kwargs)
  File "C:\projects\bitcoin\test\functional\test_framework\test_node.py", line 453, in send_cli
    raise JSONRPCException(dict(code=int(code), message=message))
test_framework.authproxy.JSONRPCException: Wallet file verification failed: Error loading wallet w2. Duplicate -wallet filename specified. (-4)
{
        entry.pushKV("blockhash", wtx.hashBlock.GetHex());
        entry.pushKV("blockindex", wtx.nIndex);
        entry.pushKV("blocktime", LookupBlockIndex(wtx.hashBlock)->GetBlockTime());
    } else {
        entry.pushKV("trusted", wtx.IsTrusted(locked_chain));
    }
    uint256 hash = wtx.GetHash();
    entry.pushKV("txid", hash.GetHex());
    UniValue conflicts(UniValue::VARR);
    for (const uint256& conflict : wtx.GetConflicts())
        conflicts.push_back(conflict.GetHex());
    entry.pushKV("walletconflicts", conflicts);
    entry.pushKV("time", wtx.GetTxTime());
    entry.pushKV("timereceived", (int64_t)wtx.nTimeReceived);
    
    std::string rbfStatus = "no";
    if (confirms <= 0) {
        LOCK(mempool.cs);
        RBFTransactionState rbfState = IsRBFOptIn(*wtx.tx, mempool);
        if (rbfState == RBFTransactionState::UNKNOWN)
            rbfStatus = "unknown";
        else if (rbfState == RBFTransactionState::REPLACEABLE_BIP125)
            rbfStatus = "yes";
    }
    entry.pushKV("bip125-replaceable", rbfStatus);
    for (const std::pair<const std::string, std::string>& item : wtx.mapValue)
        entry.pushKV(item.first, item.second);
}
static std::string LabelFromValue(const UniValue& value)