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)
mutable CCriticalSection cs_wallet;
    /** Get database handle used by this wallet. Ideally this function would
     * not be necessary.
     */
    WalletDatabase& GetDBHandle()
    {
        return *database;
    }
    /**
     * Select a set of coins such that nValueRet >= nTargetValue and at least
     * all coins from coinControl are selected; Never select unconfirmed coins
     * if they are not ours
     */
    bool SelectCoins(const std::vector<COutput>& vAvailableCoins, const CAmount& nTargetValue, std::set<CInputCoin>& setCoinsRet, CAmount& nValueRet,
                    const CCoinControl& coin_control, CoinSelectionParams& coin_selection_params, bool& bnb_used) const EXCLUSIVE_LOCKS_REQUIRED(cs_wallet);
    const WalletLocation& GetLocation() const { return m_location; }
    /** Get a name for this wallet for logging/debugging purposes.
     */
    const std::string& GetName() const { return m_location.GetName(); }
    void LoadKeyPool(int64_t nIndex, const CKeyPool &keypool) EXCLUSIVE_LOCKS_REQUIRED(cs_wallet);
    void MarkPreSplitKeys() EXCLUSIVE_LOCKS_REQUIRED(cs_wallet);
    
    std::map<CKeyID, CKeyMetadata> mapKeyMetadata GUARDED_BY(cs_wallet);
    
    std::map<CScriptID, CKeyMetadata> m_script_metadata GUARDED_BY(cs_wallet);
    typedef std::map<unsigned int, CMasterKey> MasterKeyMap;
    MasterKeyMap mapMasterKeys;
    unsigned int nMasterKeyMaxID = 0;
    
    CWallet(interfaces::Chain& chain, const WalletLocation& location, std::unique_ptr<WalletDatabase> database) : m_chain(chain), m_location(location), database(std::move(database))