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)
);
    CAmount fee;
    int change_position;
    CMutableTransaction rawTx = ConstructTransaction(request.params[0], request.params[1], request.params[2], request.params[3]["replaceable"]);
    FundTransaction(pwallet, rawTx, fee, change_position, request.params[3]);
    
    PartiallySignedTransaction psbtx(rawTx);
    
    bool bip32derivs = request.params[4].isNull() ? false : request.params[4].get_bool();
    FillPSBT(pwallet, psbtx, 1, false, bip32derivs);
    
    CDataStream ssTx(SER_NETWORK, PROTOCOL_VERSION);
    ssTx << psbtx;
    UniValue result(UniValue::VOBJ);
    result.pushKV("psbt", EncodeBase64(ssTx.str()));
    result.pushKV("fee", ValueFromAmount(fee));
    result.pushKV("changepos", change_position);
    return result;
}
UniValue abortrescan(const JSONRPCRequest& request); 
UniValue dumpprivkey(const JSONRPCRequest& request); 
UniValue importprivkey(const JSONRPCRequest& request);
UniValue importaddress(const JSONRPCRequest& request);
UniValue importpubkey(const JSONRPCRequest& request);
UniValue dumpwallet(const JSONRPCRequest& request);
UniValue importwallet(const JSONRPCRequest& request);
UniValue importprunedfunds(const JSONRPCRequest& request);
UniValue removeprunedfunds(const JSONRPCRequest& request);
UniValue importmulti(const JSONRPCRequest& request);
static const CRPCCommand commands[] =