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)
if (peerLogic) UnregisterValidationInterface(peerLogic.get());
    if (g_connman) g_connman->Stop();
    if (g_txindex) g_txindex->Stop();
    StopTorControl();
    
    
    threadGroup.interrupt_all();
    threadGroup.join_all();
    
    
    peerLogic.reset();
    g_connman.reset();
    g_txindex.reset();
    if (g_is_mempool_loaded && gArgs.GetArg("-persistmempool", DEFAULT_PERSIST_MEMPOOL)) {
        DumpMempool();
    }
    if (fFeeEstimatesInitialized)
    {
        ::feeEstimator.FlushUnconfirmed();
        fs::path est_path = GetDataDir() / FEE_ESTIMATES_FILENAME;
        CAutoFile est_fileout(fsbridge::fopen(est_path, "wb"), SER_DISK, CLIENT_VERSION);
        if (!est_fileout.IsNull())
            ::feeEstimator.Write(est_fileout);
        else
            LogPrintf("%s: Failed to write fee estimates to %s\n", __func__, est_path.string());
        fFeeEstimatesInitialized = false;
    }
    
    if (pcoinsTip != nullptr) {
        FlushStateToDisk();