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)
"    \"category\":\"send|receive\",     (string) The transaction category. 'send' has negative amounts, 'receive' has positive amounts.\n"
            "    \"amount\": x.xxx,          (numeric) The amount in " + CURRENCY_UNIT + ". This is negative for the 'send' category, and for the 'move' category for moves \n"
            "                                          outbound. It is positive for the 'receive' category, and for the 'move' category for inbound funds.\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 'send' category of transactions.\n"
            "    \"confirmations\": n,       (numeric) The number of confirmations for the transaction. Available for 'send' and 'receive' category of transactions.\n"
            "                                          When it's < 0, it means the transaction conflicted that many blocks ago.\n"
            "    \"blockhash\": \"hashvalue\",     (string) The block hash containing the transaction. Available for 'send' and 'receive' category of transactions.\n"
            "    \"blockindex\": n,          (numeric) The index of the transaction in the block that includes it. Available for 'send' and 'receive' category of transactions.\n"
            "    \"blocktime\": xxx,         (numeric) The block time in seconds since epoch (1 Jan 1970 GMT).\n"
            "    \"txid\": \"transactionid\",  (string) The transaction id. Available for 'send' and 'receive' category of transactions.\n"
            "    \"time\": xxx,              (numeric) The transaction time in seconds since epoch (Jan 1 1970 GMT).\n"
            "    \"timereceived\": xxx,      (numeric) The time received in seconds since epoch (Jan 1 1970 GMT). Available for 'send' and 'receive' category of transactions.\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 'send' category of transactions.\n"
            "    \"comment\": \"...\",       (string) If a comment is associated with the transaction.\n"
            "    \"label\" : \"label\"       (string) A comment for the address/transaction, if any\n"
            "    \"to\": \"...\",            (string) If a comment to is associated with the transaction.\n"
            "  ],\n"
            "  \"removed\": [\n"
            "    <structure is the same as \"transactions\" above, only present if include_removed=true>\n"
            "    Note: transactions that were re-added in the active chain will appear as-is in this array, and may thus have a positive confirmation count.\n"
            "  ],\n"
            "  \"lastblock\": \"lastblockhash\"     (string) The hash of the block (target_confirmations-1) from the best block on the main chain. This is typically used to feed back into listsinceblock the next time you call it. So you would generally use a target_confirmations of say 6, so you will be continually re-notified of transactions until they've reached 6 confirmations plus any new ones\n"
            "}\n"
            "\nExamples:\n"
            + HelpExampleCli("listsinceblock", "")
            + HelpExampleCli("listsinceblock", "\"000000000000000bacf66f7497b7dc45ef753ee9a7d38571037cdb1a57f663ad\" 6")
            + HelpExampleRpc("listsinceblock", "\"000000000000000bacf66f7497b7dc45ef753ee9a7d38571037cdb1a57f663ad\", 6")