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)
{
    std::shared_ptr<CWallet> const wallet = GetWalletForJSONRPCRequest(request);
    CWallet* const pwallet = wallet.get();
    if (!EnsureWalletIsAvailable(pwallet, request.fHelp)) {
        return NullUniValue;
    }
    if (request.fHelp || request.params.size() < 1 || request.params.size() > 3)
        throw std::runtime_error(
            RPCHelpMan{"fundrawtransaction",
                "\nAdd inputs to a transaction until it has enough in value to meet its out value.\n"
                "This will not modify existing inputs, and will add at most one change output to the outputs.\n"
                "No existing outputs will be modified unless \"subtractFeeFromOutputs\" is specified.\n"
                "Note that inputs which were signed may need to be resigned after completion since in/outputs have been added.\n"
                "The inputs added will not be signed, use signrawtransaction for that.\n"
                "Note that all existing inputs must have their previous output transaction be in the wallet.\n"
                "Note that all inputs selected must be of standard form and P2SH scripts must be\n"
                "in the wallet using importaddress or addmultisigaddress (to calculate fees).\n"
                "You can see whether this is the case by checking the \"solvable\" field in the listunspent output.\n"
                "Only pay-to-pubkey, multisig, and P2SH versions thereof are currently supported for watch-only\n",
                {
                    {"hexstring", RPCArg::Type::STR_HEX,  false,  "", "The hex string of the raw transaction"},
                    {"options", RPCArg::Type::OBJ,  true,  "null", "for backward compatibility: passing in a true instead of an object will result in {\"includeWatching\":true}",
                        {
                            {"changeAddress", RPCArg::Type::STR,  true,  "pool address", "The bitcoin address to receive the change"},