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)
{ 
    
    { "generating",         "generate",                         &generate,                      {"nblocks","maxtries"} },
    { "hidden",             "resendwallettransactions",         &resendwallettransactions,      {} },
    { "rawtransactions",    "fundrawtransaction",               &fundrawtransaction,            {"hexstring","options","iswitness"} },
    { "wallet",             "abandontransaction",               &abandontransaction,            {"txid"} },
    { "wallet",             "abortrescan",                      &abortrescan,                   {} },
    { "wallet",             "addmultisigaddress",               &addmultisigaddress,            {"nrequired","keys","label","address_type"} },
    { "wallet",             "backupwallet",                     &backupwallet,                  {"destination"} },
    { "wallet",             "bumpfee",                          &bumpfee,                       {"txid", "options"} },
    { "wallet",             "createwallet",                     &createwallet,                  {"wallet_name", "disable_private_keys"} },
    { "wallet",             "dumpprivkey",                      &dumpprivkey,                   {"address"}  },
    { "wallet",             "dumpwallet",                       &dumpwallet,                    {"filename"} },
    { "wallet",             "encryptwallet",                    &encryptwallet,                 {"passphrase"} },
    { "wallet",             "getaddressesbylabel",              &getaddressesbylabel,           {"label"} },
    { "wallet",             "getaddressinfo",                   &getaddressinfo,                {"address"} },
    { "wallet",             "getbalance",                       &getbalance,                    {"dummy","minconf","include_watchonly"} },
    { "wallet",             "getnewaddress",                    &getnewaddress,                 {"label","address_type"} },
    { "wallet",             "getrawchangeaddress",              &getrawchangeaddress,           {"address_type"} },
    { "wallet",             "getreceivedbyaddress",             &getreceivedbyaddress,          {"address","minconf"} },
    { "wallet",             "getreceivedbylabel",               &getreceivedbylabel,            {"label","minconf"} },
    { "wallet",             "gettransaction",                   &gettransaction,                {"txid","include_watchonly"} },
    { "wallet",             "getunconfirmedbalance",            &getunconfirmedbalance,         {} },
    { "wallet",             "getwalletinfo",                    &getwalletinfo,                 {} },
    { "wallet",             "importaddress",                    &importaddress,                 {"address","label","rescan","p2sh"} },
    { "wallet",             "importmulti",                      &importmulti,                   {"requests","options"} },
    { "wallet",             "importprivkey",                    &importprivkey,                 {"privkey","label","rescan"} },
    { "wallet",             "importprunedfunds",                &importprunedfunds,             {"rawtransaction","txoutproof"} },
    { "wallet",             "importpubkey",                     &importpubkey,                  {"pubkey","label","rescan"} },
    { "wallet",             "importwallet",                     &importwallet,                  {"filename"} },