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)
if (!EnsureWalletIsAvailable(pwallet, request.fHelp)) {
        return NullUniValue;
    }
    if (request.fHelp || request.params.size() != 0)
        throw std::runtime_error(
            RPCHelpMan{"getwalletinfo",
                "Returns an object containing various wallet state info.\n", {}}
                .ToString() +
            "\nResult:\n"
            "{\n"
            "  \"walletname\": xxxxx,               (string) the wallet name\n"
            "  \"walletversion\": xxxxx,            (numeric) the wallet version\n"
            "  \"balance\": xxxxxxx,                (numeric) the total confirmed balance of the wallet in " + CURRENCY_UNIT + "\n"
            "  \"unconfirmed_balance\": xxx,        (numeric) the total unconfirmed balance of the wallet in " + CURRENCY_UNIT + "\n"
            "  \"immature_balance\": xxxxxx,        (numeric) the total immature balance of the wallet in " + CURRENCY_UNIT + "\n"
            "  \"txcount\": xxxxxxx,                (numeric) the total number of transactions in the wallet\n"
            "  \"keypoololdest\": xxxxxx,           (numeric) the timestamp (seconds since Unix epoch) of the oldest pre-generated key in the key pool\n"
            "  \"keypoolsize\": xxxx,               (numeric) how many new keys are pre-generated (only counts external keys)\n"
            "  \"keypoolsize_hd_internal\": xxxx,   (numeric) how many new keys are pre-generated for internal use (used for change outputs, only appears if the wallet is using this feature, otherwise external keys are used)\n"
            "  \"unlocked_until\": ttt,             (numeric) the timestamp in seconds since epoch (midnight Jan 1 1970 GMT) that the wallet is unlocked for transfers, or 0 if the wallet is locked\n"
            "  \"paytxfee\": x.xxxx,                (numeric) the transaction fee configuration, set in " + CURRENCY_UNIT + "/kB\n"
            "  \"hdseedid\": \"<hash160>\"            (string, optional) the Hash160 of the HD seed (only present when HD is enabled)\n"
            "  \"hdmasterkeyid\": \"<hash160>\"       (string, optional) alias for hdseedid retained for backwards-compatibility. Will be removed in V0.18.\n"
            "  \"private_keys_enabled\": true|false (boolean) false if privatekeys are disabled for this wallet (enforced watch-only wallet)\n"
            "}\n"
            "\nExamples:\n"
            + HelpExampleCli("getwalletinfo", "")
            + HelpExampleRpc("getwalletinfo", "")
        );