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)
dustRelayFee = CFeeRate(n);
    }
    fRequireStandard = !gArgs.GetBoolArg("-acceptnonstdtxn", !chainparams.RequireStandard());
    if (chainparams.RequireStandard() && !fRequireStandard)
        return InitError(strprintf("acceptnonstdtxn is not currently supported for %s chain", chainparams.NetworkIDString()));
    nBytesPerSigOp = gArgs.GetArg("-bytespersigop", nBytesPerSigOp);
    if (!g_wallet_init_interface.ParameterInteraction()) return false;
    fIsBareMultisigStd = gArgs.GetBoolArg("-permitbaremultisig", DEFAULT_PERMIT_BAREMULTISIG);
    fAcceptDatacarrier = gArgs.GetBoolArg("-datacarrier", DEFAULT_ACCEPT_DATACARRIER);
    nMaxDatacarrierBytes = gArgs.GetArg("-datacarriersize", nMaxDatacarrierBytes);
    
    SetMockTime(gArgs.GetArg("-mocktime", 0)); 
    if (gArgs.GetBoolArg("-peerbloomfilters", DEFAULT_PEERBLOOMFILTERS))
        nLocalServices = ServiceFlags(nLocalServices | NODE_BLOOM);
    if (gArgs.GetArg("-rpcserialversion", DEFAULT_RPC_SERIALIZE_VERSION) < 0)
        return InitError("rpcserialversion must be non-negative.");
    if (gArgs.GetArg("-rpcserialversion", DEFAULT_RPC_SERIALIZE_VERSION) > 1)
        return InitError("unknown rpcserialversion requested.");
    nMaxTipAge = gArgs.GetArg("-maxtipage", DEFAULT_MAX_TIP_AGE);
    fEnableReplacement = gArgs.GetBoolArg("-mempoolreplacement", DEFAULT_ENABLE_REPLACEMENT);
    if ((!fEnableReplacement) && gArgs.IsArgSet("-mempoolreplacement")) {
        
        std::string strReplacementModeList = gArgs.GetArg("-mempoolreplacement", "");  
        std::vector<std::string> vstrReplacementModes;
        boost::split(vstrReplacementModes, strReplacementModeList, boost::is_any_of(","));
        fEnableReplacement = (std::find(vstrReplacementModes.begin(), vstrReplacementModes.end(), "fee") != vstrReplacementModes.end());
    }
    return true;
}
static bool LockDataDirectory(bool probeOnly)