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)
pwallet->BlockUntilSyncedToCurrentChain();
    auto locked_chain = pwallet->chain().lock();
    LOCK(pwallet->cs_wallet);
    uint256 hash(ParseHashV(request.params[0], "txid"));
    isminefilter filter = ISMINE_SPENDABLE;
    if(!request.params[1].isNull())
        if(request.params[1].get_bool())
            filter = filter | ISMINE_WATCH_ONLY;
    UniValue entry(UniValue::VOBJ);
    auto it = pwallet->mapWallet.find(hash);
    if (it == pwallet->mapWallet.end()) {
        throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "Invalid or non-wallet transaction id");
    }
    const CWalletTx& wtx = it->second;
    CAmount nCredit = wtx.GetCredit(*locked_chain, filter);
    CAmount nDebit = wtx.GetDebit(filter);
    CAmount nNet = nCredit - nDebit;
    CAmount nFee = (wtx.IsFromMe(filter) ? wtx.tx->GetValueOut() - nDebit : 0);
    entry.pushKV("amount", ValueFromAmount(nNet - nFee));
    if (wtx.IsFromMe(filter))
        entry.pushKV("fee", ValueFromAmount(nFee));
    WalletTxToJSON(pwallet->chain(), *locked_chain, wtx, entry);
    UniValue details(UniValue::VARR);
    ListTransactions(*locked_chain, pwallet, wtx, 0, false, details, filter, nullptr );
    entry.pushKV("details", details);
    std::string strHex = EncodeHexTx(*wtx.tx, RPCSerializationFlags());
    entry.pushKV("hex", strHex);
    return entry;
}
static UniValue abandontransaction(const JSONRPCRequest& request)