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)
{
                            {"", RPCArg::Type::OBJ,  true,  "", "",
                                {
                                    {"address", RPCArg::Type::AMOUNT,  false,  "", "A key-value pair. The key (string) is the bitcoin address, the value (float or string) is the amount in " + CURRENCY_UNIT + ""},
                                },
                                },
                            {"", RPCArg::Type::OBJ,  true,  "", "",
                                {
                                    {"data", RPCArg::Type::STR_HEX,  false,  "", "A key-value pair. The key must be \"data\", the value is hex-encoded data"},
                                },
                            },
                        },
                    },
                    {"locktime", RPCArg::Type::NUM,  true,  "0", "Raw locktime. Non-0 value also locktime-activates inputs\n"
                            "                             Allows this transaction to be replaced by a transaction with higher fees. If provided, it is an error if explicit sequence numbers are incompatible."},
                    {"options", RPCArg::Type::OBJ,  true,  "null", "",
                        {
                            {"changeAddress", RPCArg::Type::STR_HEX,  true,  "pool address", "The bitcoin address to receive the change"},
                            {"changePosition", RPCArg::Type::NUM,  true,  "random", "The index of the change output"},
                            {"change_type", RPCArg::Type::STR,  true,  "set by -changetype", "The output type to use. Only valid if changeAddress is not specified. Options are \"legacy\", \"p2sh-segwit\", and \"bech32\"."},
                            {"includeWatching", RPCArg::Type::BOOL,  true,  "false", "Also select inputs which are watch only"},
                            {"lockUnspents", RPCArg::Type::BOOL,  true,  "false", "Lock selected unspent outputs"},
                            {"feeRate", RPCArg::Type::AMOUNT,  true,  "not set: makes wallet determine the fee", "Set a specific fee rate in " + CURRENCY_UNIT + "/kB"},
                            {"subtractFeeFromOutputs", RPCArg::Type::ARR,  true,  "empty array", "A json array of integers.\n"
                            "                              The fee will be equally deducted from the amount of each specified output.\n"
                            "                              Those recipients will receive less bitcoins than you enter in their corresponding amount field.\n"
                            "                              If no outputs are specified here, the sender pays the fee.",
                                {
                                    {"vout_index", RPCArg::Type::NUM,  true,  "", "The zero-based output index, before a change output is added."},
                                },