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)
int depth = it->second.GetDepthInMainChain(locked_chain);
            if (depth >= 0 && output.n < it->second.tx->vout.size() &&
                IsMine(it->second.tx->vout[output.n]) == ISMINE_SPENDABLE) {
                CTxDestination address;
                if (ExtractDestination(FindNonChangeParentOutput(*it->second.tx, output.n).scriptPubKey, address)) {
                    result[address].emplace_back(
                        &it->second, output.n, depth, true , true , false );
                }
            }
        }
    }
    return result;
}
const CTxOut& CWallet::FindNonChangeParentOutput(const CTransaction& tx, int output) const
{
    const CTransaction* ptx = &tx;
    int n = output;
    while (IsChange(ptx->vout[n]) && ptx->vin.size() > 0) {
        const COutPoint& prevout = ptx->vin[0].prevout;
        auto it = mapWallet.find(prevout.hash);
        if (it == mapWallet.end() || it->second.tx->vout.size() <= prevout.n ||
            !IsMine(it->second.tx->vout[prevout.n])) {
            break;
        }
        ptx = it->second.tx.get();
        n = prevout.n;
    }
    return ptx->vout[n];
}
bool CWallet::SelectCoinsMinConf(const CAmount& nTargetValue, const CoinEligibilityFilter& eligibility_filter, std::vector<OutputGroup> groups,