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)
"[\n"
            "  {\n"
            "    \"address\":\"address\",    (string) The bitcoin address of the transaction.\n"
            "    \"category\":\"send|receive\", (string) The transaction category.\n"
            "    \"amount\": x.xxx,          (numeric) The amount in " + CURRENCY_UNIT + ". This is negative for the 'send' category, and is positive\n"
            "                                        for the 'receive' category,\n"
            "    \"label\": \"label\",       (string) A comment for the address/transaction, if any\n"
            "    \"vout\": n,                (numeric) the vout value\n"
            "    \"fee\": x.xxx,             (numeric) The amount of the fee in " + CURRENCY_UNIT + ". This is negative and only available for the \n"
            "                                         'send' category of transactions.\n"
            "    \"confirmations\": n,       (numeric) The number of confirmations for the transaction. Negative confirmations indicate the\n"
            "                                         transaction conflicts with the block chain\n"
            "    \"trusted\": xxx,           (bool) Whether we consider the outputs of this unconfirmed transaction safe to spend.\n"
            "    \"blockhash\": \"hashvalue\", (string) The block hash containing the transaction.\n"
            "    \"blockindex\": n,          (numeric) The index of the transaction in the block that includes it.\n"
            "    \"blocktime\": xxx,         (numeric) The block time in seconds since epoch (1 Jan 1970 GMT).\n"
            "    \"txid\": \"transactionid\", (string) The transaction id.\n"
            "    \"time\": xxx,              (numeric) The transaction time in seconds since epoch (midnight Jan 1 1970 GMT).\n"
            "    \"timereceived\": xxx,      (numeric) The time received in seconds since epoch (midnight Jan 1 1970 GMT).\n"
            "    \"comment\": \"...\",       (string) If a comment is associated with the transaction.\n"
            "    \"bip125-replaceable\": \"yes|no|unknown\",  (string) Whether this transaction could be replaced due to BIP125 (replace-by-fee);\n"
            "                                                     may be unknown for unconfirmed transactions not in the mempool\n"
            "    \"abandoned\": xxx          (bool) 'true' if the transaction has been abandoned (inputs are respendable). Only available for the \n"
            "                                         'send' category of transactions.\n"
            "  }\n"
            "]\n"
            "\nExamples:\n"
            "\nList the most recent 10 transactions in the systems\n"
            + HelpExampleCli("listtransactions", "") +
            "\nList transactions 100 to 120\n"