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)
"Note: An output is discarded if it is dust at this rate, but we will always discard up to the dust relay fee and a discard fee above that is limited by the fee estimate for the longest target",
                                                              CURRENCY_UNIT, FormatMoney(DEFAULT_DISCARD_FEE)), false, OptionsCategory::WALLET);
    gArgs.AddArg("-fallbackfee=<amt>", strprintf("A fee rate (in %s/kB) that will be used when fee estimation has insufficient data (default: %s)",
                                                               CURRENCY_UNIT, FormatMoney(DEFAULT_FALLBACK_FEE)), false, OptionsCategory::WALLET);
    gArgs.AddArg("-keypool=<n>", strprintf("Set key pool size to <n> (default: %u)", DEFAULT_KEYPOOL_SIZE), false, OptionsCategory::WALLET);
    gArgs.AddArg("-mintxfee=<amt>", strprintf("Fees (in %s/kB) smaller than this are considered zero fee for transaction creation (default: %s)",
                                                            CURRENCY_UNIT, FormatMoney(DEFAULT_TRANSACTION_MINFEE)), false, OptionsCategory::WALLET);
    gArgs.AddArg("-paytxfee=<amt>", strprintf("Fee (in %s/kB) to add to transactions you send (default: %s)",
                                                            CURRENCY_UNIT, FormatMoney(CFeeRate{DEFAULT_PAY_TX_FEE}.GetFeePerK())), false, OptionsCategory::WALLET);
    gArgs.AddArg("-rescan", "Rescan the block chain for missing wallet transactions on startup", false, OptionsCategory::WALLET);
    gArgs.AddArg("-salvagewallet", "Attempt to recover private keys from a corrupt wallet on startup", false, OptionsCategory::WALLET);
    gArgs.AddArg("-spendzeroconfchange", strprintf("Spend unconfirmed change when sending transactions (default: %u)", DEFAULT_SPEND_ZEROCONF_CHANGE), false, OptionsCategory::WALLET);
    gArgs.AddArg("-txconfirmtarget=<n>", strprintf("If paytxfee is not set, include enough fee so transactions begin confirmation on average within n blocks (default: %u)", DEFAULT_TX_CONFIRM_TARGET), false, OptionsCategory::WALLET);
    gArgs.AddArg("-upgradewallet", "Upgrade wallet to latest format on startup", false, OptionsCategory::WALLET);
    gArgs.AddArg("-wallet=<path>", "Specify wallet database path. Can be specified multiple times to load multiple wallets. Path is interpreted relative to <walletdir> if it is not absolute, and will be created if it does not exist (as a directory containing a wallet.dat file and log files). For backwards compatibility this will also accept names of existing data files in <walletdir>.)", false, OptionsCategory::WALLET);
    gArgs.AddArg("-walletbroadcast",  strprintf("Make the wallet broadcast transactions (default: %u)", DEFAULT_WALLETBROADCAST), false, OptionsCategory::WALLET);
    gArgs.AddArg("-walletdir=<dir>", "Specify directory to hold wallets (default: <datadir>/wallets if it exists, otherwise <datadir>)", false, OptionsCategory::WALLET);
    gArgs.AddArg("-walletnotify=<cmd>", "Execute command when a wallet transaction changes (%s in cmd is replaced by TxID)", false, OptionsCategory::WALLET);
    gArgs.AddArg("-walletrbf", strprintf("Send transactions with full-RBF opt-in enabled (RPC only, default: %u)", DEFAULT_WALLET_RBF), false, OptionsCategory::WALLET);
    gArgs.AddArg("-zapwallettxes=<mode>", "Delete all wallet transactions and only recover those parts of the blockchain through -rescan on startup"
                               " (1 = keep tx meta data e.g. payment request information, 2 = drop tx meta data)", false, OptionsCategory::WALLET);
    gArgs.AddArg("-dblogsize=<n>", strprintf("Flush wallet database activity from memory to disk log every <n> megabytes (default: %u)", DEFAULT_WALLET_DBLOGSIZE), true, OptionsCategory::WALLET_DEBUG_TEST);
    gArgs.AddArg("-flushwallet", strprintf("Run a thread to flush wallet periodically (default: %u)", DEFAULT_FLUSHWALLET), true, OptionsCategory::WALLET_DEBUG_TEST);
    gArgs.AddArg("-privdb", strprintf("Sets the DB_PRIVATE flag in the wallet db environment (default: %u)", DEFAULT_WALLET_PRIVDB), true, OptionsCategory::WALLET_DEBUG_TEST);
    gArgs.AddArg("-walletrejectlongchains", strprintf("Wallet will not create transactions that violate mempool chain limits (default: %u)", DEFAULT_WALLET_REJECT_LONG_CHAINS), true, OptionsCategory::WALLET_DEBUG_TEST);
}
bool WalletInit::ParameterInteraction() const
{
    if (gArgs.GetBoolArg("-disablewallet", DEFAULT_DISABLE_WALLET)) {
        for (const std::string& wallet : gArgs.GetArgs("-wallet")) {