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)
{
private:
    std::atomic<bool> fAbortRescan{false};
    std::atomic<bool> fScanningWallet{false}; 
    std::mutex mutexScanning;
    friend class WalletRescanReserver;
    WalletBatch *encrypted_batch GUARDED_BY(cs_wallet) = nullptr;
    
    int nWalletVersion = FEATURE_BASE;
    
    int nWalletMaxVersion GUARDED_BY(cs_wallet) = FEATURE_BASE;
    int64_t nNextResend = 0;
    int64_t nLastResend = 0;
    bool fBroadcastTransactions = false;
    /**
     * Used to keep track of spent outpoints, and
     * detect and report conflicts (double-spends or
     * mutated transactions where the mutant gets mined).
     */
    typedef std::multimap<COutPoint, uint256> TxSpends;
    TxSpends mapTxSpends GUARDED_BY(cs_wallet);
    void AddToSpends(const COutPoint& outpoint, const uint256& wtxid) EXCLUSIVE_LOCKS_REQUIRED(cs_wallet);
    void AddToSpends(const uint256& wtxid) EXCLUSIVE_LOCKS_REQUIRED(cs_wallet);
    /**
     * Add a transaction to the wallet, or update it.  pIndex and posInBlock should
     * be set when the transaction was known to be included in a block.  When
     * pIndex == nullptr, then wallet state is not updated in AddToWallet, but
     * notifications happen and cached balances are marked dirty.