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)
void Serialize(Stream& s) const
    {
        char fSpent = false;
        mapValue_t mapValueCopy = mapValue;
        mapValueCopy["fromaccount"] = "";
        WriteOrderPos(nOrderPos, mapValueCopy);
        if (nTimeSmart) {
            mapValueCopy["timesmart"] = strprintf("%u", nTimeSmart);
        }
        s << static_cast<const CMerkleTx&>(*this);
        std::vector<CMerkleTx> vUnused; 
        s << vUnused << mapValueCopy << vOrderForm << fTimeReceivedIsTxTime << nTimeReceived << fFromMe << fSpent;
    }
    template<typename Stream>
    void Unserialize(Stream& s)
    {
        Init(nullptr);
        char fSpent;
        s >> static_cast<CMerkleTx&>(*this);
        std::vector<CMerkleTx> vUnused; 
        s >> vUnused >> mapValue >> vOrderForm >> fTimeReceivedIsTxTime >> nTimeReceived >> fFromMe >> fSpent;
        ReadOrderPos(nOrderPos, mapValue);
        nTimeSmart = mapValue.count("timesmart") ? (unsigned int)atoi64(mapValue["timesmart"]) : 0;
        mapValue.erase("fromaccount");
        mapValue.erase("spent");
        mapValue.erase("n");
        mapValue.erase("timesmart");
    }
    
    void MarkDirty()