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)
{
    nIndex = -1;
    keypool.vchPubKey = CPubKey();
    {
        LOCK(cs_wallet);
        if (!IsLocked())
            TopUpKeyPool();
        bool fReturningInternal = IsHDEnabled() && CanSupportFeature(FEATURE_HD_SPLIT) && fRequestedInternal;
        bool use_split_keypool = set_pre_split_keypool.empty();
        std::set<int64_t>& setKeyPool = use_split_keypool ? (fReturningInternal ? setInternalKeyPool : setExternalKeyPool) : set_pre_split_keypool;
        
        if (setKeyPool.empty()) {
            return false;
        }
        WalletBatch batch(*database);
        auto it = setKeyPool.begin();
        nIndex = *it;
        setKeyPool.erase(it);
        if (!batch.ReadPool(nIndex, keypool)) {
            throw std::runtime_error(std::string(__func__) + ": read failed");
        }
        if (!HaveKey(keypool.vchPubKey.GetID())) {
            throw std::runtime_error(std::string(__func__) + ": unknown key in key pool");
        }
        
        if (use_split_keypool && keypool.fInternal != fReturningInternal) {
            throw std::runtime_error(std::string(__func__) + ": keypool entry misclassified");
        }
        if (!keypool.vchPubKey.IsValid()) {
            throw std::runtime_error(std::string(__func__) + ": keypool entry invalid");