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)
bool IsHDEnabled() const;
    
    CPubKey GenerateNewSeed();
    
    CPubKey DeriveNewSeed(const CKey& key);
    /* Set the current HD seed (will reset the chain child index counters)
       Sets the seed's version based on the current wallet version (so the
       caller must ensure the current wallet version is correct before calling
       this function). */
    void SetHDSeed(const CPubKey& key);
    /**
     * Blocks until the wallet state is up-to-date to /at least/ the current
     * chain at the time this function is entered
     * Obviously holding cs_main/cs_wallet when going into this call may cause
     * deadlock
     */
    void BlockUntilSyncedToCurrentChain() LOCKS_EXCLUDED(cs_main, cs_wallet);
    /**
     * Explicitly make the wallet learn the related scripts for outputs to the
     * given key. This is purely to make the wallet file compatible with older
     * software, as CBasicKeyStore automatically does this implicitly for all
     * keys now.
     */
    void LearnRelatedScripts(const CPubKey& key, OutputType);
    /**
     * Same as LearnRelatedScripts, but when the OutputType is not known (and could
     * be anything).
     */