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)
#include <string>
#include <utility>
#include <vector>
bool VerifyWallets(interfaces::Chain& chain, const std::vector<std::string>& wallet_files);
bool LoadWallets(interfaces::Chain& chain, const std::vector<std::string>& wallet_files);
void StartWallets(CScheduler& scheduler);
void FlushWallets();
void StopWallets();
void UnloadWallets();
void UnloadWallet(std::shared_ptr<CWallet>&& wallet);
bool AddWallet(const std::shared_ptr<CWallet>& wallet);
bool RemoveWallet(const std::shared_ptr<CWallet>& wallet);
bool HasWallets();
std::vector<std::shared_ptr<CWallet>> GetWallets();
std::shared_ptr<CWallet> GetWallet(const std::string& name);
static const unsigned int DEFAULT_KEYPOOL_SIZE = 1000;
constexpr CAmount DEFAULT_PAY_TX_FEE = 0;
static const CAmount DEFAULT_FALLBACK_FEE = 20000;
static const CAmount DEFAULT_DISCARD_FEE = 10000;
static const CAmount DEFAULT_TRANSACTION_MINFEE = 1000;
static const CAmount WALLET_INCREMENTAL_RELAY_FEE = 5000;
static const bool DEFAULT_SPEND_ZEROCONF_CHANGE = true;
static const bool DEFAULT_WALLET_REJECT_LONG_CHAINS = false;
static const bool DEFAULT_AVOIDPARTIALSPENDS = false;
static const unsigned int DEFAULT_TX_CONFIRM_TARGET = 6;
static const bool DEFAULT_WALLET_RBF = false;
static const bool DEFAULT_WALLETBROADCAST = true;
static const bool DEFAULT_DISABLE_WALLET = false;
static constexpr size_t DUMMY_NESTED_P2WPKH_INPUT_SIZE = 91;
class CBlockIndex;