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)
CScript inner = CreateMultisigRedeemscript(required, pubkeys);
    CTxDestination dest = AddAndGetDestinationForScript(*pwallet, inner, output_type);
    pwallet->SetAddressBook(dest, label, "send");
    UniValue result(UniValue::VOBJ);
    result.pushKV("address", EncodeDestination(dest));
    result.pushKV("redeemScript", HexStr(inner.begin(), inner.end()));
    return result;
}
struct tallyitem
{
    CAmount nAmount;
    int nConf;
    std::vector<uint256> txids;
    bool fIsWatchonly;
    tallyitem()
    {
        nAmount = 0;
        nConf = std::numeric_limits<int>::max();
        fIsWatchonly = false;
    }
};
static UniValue ListReceived(interfaces::Chain::Lock& locked_chain, CWallet * const pwallet, const UniValue& params, bool by_label) EXCLUSIVE_LOCKS_REQUIRED(pwallet->cs_wallet)
{
    LockAnnotation lock(::cs_main); 
    
    int nMinDepth = 1;
    if (!params[0].isNull())
        nMinDepth = params[0].get_int();