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)
std::string error_string;
        std::string warning_string;
        bool verify_success = CWallet::Verify(chain, location, salvage_wallet, error_string, warning_string);
        if (!error_string.empty()) InitError(error_string);
        if (!warning_string.empty()) InitWarning(warning_string);
        if (!verify_success) return false;
    }
    return true;
}
void WalletInit::Construct(InitInterfaces& interfaces) const
{
    if (gArgs.GetBoolArg("-disablewallet", DEFAULT_DISABLE_WALLET)) {
        LogPrintf("Wallet disabled!\n");
        return;
    }
    gArgs.SoftSetArg("-wallet", "");
    interfaces.chain_clients.emplace_back(interfaces::MakeWalletClient(*interfaces.chain, gArgs.GetArgs("-wallet")));
}
bool LoadWallets(interfaces::Chain& chain, const std::vector<std::string>& wallet_files)
{
    for (const std::string& walletFile : wallet_files) {
        std::shared_ptr<CWallet> pwallet = CWallet::CreateWalletFromFile(chain, WalletLocation(walletFile));
        if (!pwallet) {
            return false;
        }
        AddWallet(pwallet);
    }
    return true;
}
void StartWallets(CScheduler& scheduler)