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)
.ToString() +
            "\nResult:\n"
            "\"address\"    (string) The address\n"
            "\nExamples:\n"
            + HelpExampleCli("getrawchangeaddress", "")
            + HelpExampleRpc("getrawchangeaddress", "")
       );
    if (pwallet->IsWalletFlagSet(WALLET_FLAG_DISABLE_PRIVATE_KEYS)) {
        throw JSONRPCError(RPC_WALLET_ERROR, "Error: Private keys are disabled for this wallet");
    }
    LOCK(pwallet->cs_wallet);
    if (!pwallet->IsLocked()) {
        pwallet->TopUpKeyPool();
    }
    OutputType output_type = pwallet->m_default_change_type != OutputType::CHANGE_AUTO ? pwallet->m_default_change_type : pwallet->m_default_address_type;
    if (!request.params[0].isNull()) {
        if (!ParseOutputType(request.params[0].get_str(), output_type)) {
            throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, strprintf("Unknown address type '%s'", request.params[0].get_str()));
        }
    }
    CReserveKey reservekey(pwallet);
    CPubKey vchPubKey;
    if (!reservekey.GetReservedKey(vchPubKey, true))
        throw JSONRPCError(RPC_WALLET_KEYPOOL_RAN_OUT, "Error: Keypool ran out, please call keypoolrefill first");
    reservekey.KeepKey();
    pwallet->LearnRelatedScripts(vchPubKey, output_type);
    CTxDestination dest = GetDestinationForKey(vchPubKey, output_type);
    return EncodeDestination(dest);
}
static UniValue setlabel(const JSONRPCRequest& request)