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)
};
    ScanResult ScanForWalletTransactions(const CBlockIndex* const pindexStart, const CBlockIndex* const pindexStop, const WalletRescanReserver& reserver, const CBlockIndex*& failed_block, const CBlockIndex*& stop_block, bool fUpdate = false);
    void TransactionRemovedFromMempool(const CTransactionRef &ptx) override;
    void ReacceptWalletTransactions();
    void ResendWalletTransactions(int64_t nBestBlockTime, CConnman* connman) override EXCLUSIVE_LOCKS_REQUIRED(cs_main);
    
    std::vector<uint256> ResendWalletTransactionsBefore(interfaces::Chain::Lock& locked_chain, int64_t nTime, CConnman* connman);
    CAmount GetBalance(const isminefilter& filter=ISMINE_SPENDABLE, const int min_depth=0) const;
    CAmount GetUnconfirmedBalance() const;
    CAmount GetImmatureBalance() const;
    CAmount GetUnconfirmedWatchOnlyBalance() const;
    CAmount GetImmatureWatchOnlyBalance() const;
    CAmount GetLegacyBalance(const isminefilter& filter, int minDepth) const;
    CAmount GetAvailableBalance(const CCoinControl* coinControl = nullptr) const;
    OutputType TransactionChangeType(OutputType change_type, const std::vector<CRecipient>& vecSend);
    /**
     * Insert additional inputs into the transaction by
     * calling CreateTransaction();
     */
    bool FundTransaction(CMutableTransaction& tx, CAmount& nFeeRet, int& nChangePosInOut, std::string& strFailReason, bool lockUnspents, const std::set<int>& setSubtractFeeFromOutputs, CCoinControl);
    bool SignTransaction(CMutableTransaction& tx) EXCLUSIVE_LOCKS_REQUIRED(cs_wallet);
    /**
     * Create a new transaction paying the recipients with a set of coins
     * selected by SelectCoins(); Also create the change output, when needed
     * @note passing nChangePosInOut as -1 will result in setting a random position
     */
    bool CreateTransaction(interfaces::Chain::Lock& locked_chain, const std::vector<CRecipient>& vecSend, CTransactionRef& tx, CReserveKey& reservekey, CAmount& nFeeRet, int& nChangePosInOut,
                           std::string& strFailReason, const CCoinControl& coin_control, bool sign = true);
    bool CommitTransaction(CTransactionRef tx, mapValue_t mapValue, std::vector<std::pair<std::string, std::string>> orderForm, CReserveKey& reservekey, CConnman* connman, CValidationState& state);
    bool DummySignTx(CMutableTransaction &txNew, const std::set<CTxOut> &txouts, bool use_max_sig = false) const