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)
"Optionally filter to only include txouts paid to specified addresses.\n",
                {
                    {"minconf", RPCArg::Type::NUM,  true,  "1", "The minimum confirmations to filter"},
                    {"maxconf", RPCArg::Type::NUM,  true,  "9999999", "The maximum confirmations to filter"},
                    {"addresses", RPCArg::Type::ARR,  true,  "empty array", "A json array of bitcoin addresses to filter",
                        {
                            {"address", RPCArg::Type::STR,  true,  "", "bitcoin address"},
                        },
                    },
                    {"include_unsafe", RPCArg::Type::BOOL,  true,  "true", "Include outputs that are not safe to spend\n"
            "                  See description of \"safe\" attribute below."},
                    {"query_options", RPCArg::Type::OBJ,  true,  "null", "JSON with query options",
                        {
                            {"minimumAmount", RPCArg::Type::AMOUNT,  true,  "0", "Minimum value of each UTXO in " + CURRENCY_UNIT + ""},
                            {"maximumAmount", RPCArg::Type::AMOUNT,  true,  "unlimited", "Maximum value of each UTXO in " + CURRENCY_UNIT + ""},
                            {"maximumCount", RPCArg::Type::NUM,  true,  "unlimited", "Maximum number of UTXOs"},
                            {"minimumSumAmount", RPCArg::Type::AMOUNT,  true,  "unlimited", "Minimum sum value of all UTXOs in " + CURRENCY_UNIT + ""},
                        },
                        "query_options"},
                }}
                .ToString() +
            "\nResult\n"
            "[                   (array of json object)\n"
            "  {\n"
            "    \"txid\" : \"txid\",          (string) the transaction id \n"
            "    \"vout\" : n,               (numeric) the vout value\n"
            "    \"address\" : \"address\",    (string) the bitcoin address\n"
            "    \"label\" : \"label\",        (string) The associated label, or \"\" for the default label\n"
            "    \"scriptPubKey\" : \"key\",   (string) the script key\n"
            "    \"amount\" : x.xxx,         (numeric) the transaction output amount in " + CURRENCY_UNIT + "\n"