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)
walletInstance->SetWalletFlag(WALLET_FLAG_DISABLE_PRIVATE_KEYS);
        } else {
            
            CPubKey seed = walletInstance->GenerateNewSeed();
            walletInstance->SetHDSeed(seed);
        }
        
        if (!walletInstance->IsWalletFlagSet(WALLET_FLAG_DISABLE_PRIVATE_KEYS) && !walletInstance->TopUpKeyPool()) {
            InitError(_("Unable to generate initial keys"));
            return nullptr;
        }
        auto locked_chain = chain.assumeLocked();  
        walletInstance->ChainStateFlushed(chainActive.GetLocator());
    } else if (wallet_creation_flags & WALLET_FLAG_DISABLE_PRIVATE_KEYS) {
        
        InitError(strprintf(_("Error loading %s: Private keys can only be disabled during creation"), walletFile));
        return NULL;
    } else if (walletInstance->IsWalletFlagSet(WALLET_FLAG_DISABLE_PRIVATE_KEYS)) {
        LOCK(walletInstance->cs_KeyStore);
        if (!walletInstance->mapKeys.empty() || !walletInstance->mapCryptedKeys.empty()) {
            InitWarning(strprintf(_("Warning: Private keys detected in wallet {%s} with disabled private keys"), walletFile));
        }
    }
    if (!gArgs.GetArg("-addresstype", "").empty() && !ParseOutputType(gArgs.GetArg("-addresstype", ""), walletInstance->m_default_address_type)) {
        InitError(strprintf("Unknown address type '%s'", gArgs.GetArg("-addresstype", "")));
        return nullptr;
    }
    if (!gArgs.GetArg("-changetype", "").empty() && !ParseOutputType(gArgs.GetArg("-changetype", ""), walletInstance->m_default_change_type)) {
        InitError(strprintf("Unknown change type '%s'", gArgs.GetArg("-changetype", "")));