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)
if (startBlock) {
        const CBlockIndex *failedBlock, *stop_block;
        
        if (ScanResult::FAILURE == ScanForWalletTransactions(startBlock, nullptr, reserver, failedBlock, stop_block, update)) {
            return failedBlock->GetBlockTimeMax() + TIMESTAMP_WINDOW + 1;
        }
    }
    return startTime;
}
/**
 * Scan the block chain (starting in pindexStart) for transactions
 * from or to us. If fUpdate is true, found transactions that already
 * exist in the wallet will be updated.
 *
 * @param[in] pindexStop if not a nullptr, the scan will stop at this block-index
 * @param[out] failed_block if FAILURE is returned, the most recent block
 *     that could not be scanned, otherwise nullptr
 * @param[out] stop_block the most recent block that could be scanned,
 *     otherwise nullptr if no block could be scanned
 *
 * @return ScanResult indicating success or failure of the scan. SUCCESS if
 * scan was successful. FAILURE if a complete rescan was not possible (due to
 * pruning or corruption). USER_ABORT if the rescan was aborted before it
 * could complete.
 *
 * @pre Caller needs to make sure pindexStop (and the optional pindexStart) are on
 * the main chain after to the addition of any new keys you want to detect
 * transactions for.
 */
CWallet::ScanResult CWallet::ScanForWalletTransactions(const CBlockIndex* const pindexStart, const CBlockIndex* const pindexStop, const WalletRescanReserver& reserver, const CBlockIndex*& failed_block, const CBlockIndex*& stop_block, bool fUpdate)