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)
"  \"witness_program\" : \"hex\"     (string, optional) The hex value of the witness program\n"
            "  \"script\" : \"type\"             (string, optional) The output script type. Only if \"isscript\" is true and the redeemscript is known. Possible types: nonstandard, pubkey, pubkeyhash, scripthash, multisig, nulldata, witness_v0_keyhash, witness_v0_scripthash, witness_unknown\n"
            "  \"hex\" : \"hex\",                (string, optional) The redeemscript for the p2sh address\n"
            "  \"pubkeys\"                     (string, optional) Array of pubkeys associated with the known redeemscript (only if \"script\" is \"multisig\")\n"
            "    [\n"
            "      \"pubkey\"\n"
            "      ,...\n"
            "    ]\n"
            "  \"sigsrequired\" : xxxxx        (numeric, optional) Number of signatures required to spend multisig output (only if \"script\" is \"multisig\")\n"
            "  \"pubkey\" : \"publickeyhex\",    (string, optional) The hex value of the raw public key, for single-key addresses (possibly embedded in P2SH or P2WSH)\n"
            "  \"embedded\" : {...},           (object, optional) Information about the address embedded in P2SH or P2WSH, if relevant and known. It includes all getaddressinfo output fields for the embedded address, excluding metadata (\"timestamp\", \"hdkeypath\", \"hdseedid\") and relation to the wallet (\"ismine\", \"iswatchonly\").\n"
            "  \"iscompressed\" : true|false,  (boolean) If the address is compressed\n"
            "  \"label\" :  \"label\"         (string) The label associated with the address, \"\" is the default label\n"
            "  \"timestamp\" : timestamp,      (number, optional) The creation time of the key if available in seconds since epoch (Jan 1 1970 GMT)\n"
            "  \"hdkeypath\" : \"keypath\"       (string, optional) The HD keypath if the key is HD and available\n"
            "  \"hdseedid\" : \"<hash160>\"      (string, optional) The Hash160 of the HD seed\n"
            "  \"hdmasterkeyid\" : \"<hash160>\" (string, optional) alias for hdseedid maintained for backwards compatibility. Will be removed in V0.18.\n"
            "  \"labels\"                      (object) Array of labels associated with the address.\n"
            "    [\n"
            "      { (json object of label data)\n"
            "        \"name\": \"labelname\" (string) The label\n"
            "        \"purpose\": \"string\" (string) Purpose of address (\"send\" for sending address, \"receive\" for receiving address)\n"
            "      },...\n"
            "    ]\n"
            "}\n"
            "\nExamples:\n"
            + HelpExampleCli("getaddressinfo", "\"1PSSGeFHDnKNxiEyFrD1wcEaHr9hrQDDWc\"")
            + HelpExampleRpc("getaddressinfo", "\"1PSSGeFHDnKNxiEyFrD1wcEaHr9hrQDDWc\"")
        );
    }