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)
+ HelpExampleCli("rescanblockchain", "100000 120000")
            + HelpExampleRpc("rescanblockchain", "100000, 120000")
            );
    }
    WalletRescanReserver reserver(pwallet);
    if (!reserver.reserve()) {
        throw JSONRPCError(RPC_WALLET_ERROR, "Wallet is currently rescanning. Abort existing rescan or wait.");
    }
    CBlockIndex *pindexStart = nullptr;
    CBlockIndex *pindexStop = nullptr;
    CBlockIndex *pChainTip = nullptr;
    {
        auto locked_chain = pwallet->chain().lock();
        pindexStart = chainActive.Genesis();
        pChainTip = chainActive.Tip();
        if (!request.params[0].isNull()) {
            pindexStart = chainActive[request.params[0].get_int()];
            if (!pindexStart) {
                throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid start_height");
            }
        }
        if (!request.params[1].isNull()) {
            pindexStop = chainActive[request.params[1].get_int()];
            if (!pindexStop) {
                throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid stop_height");
            }
            else if (pindexStop->nHeight < pindexStart->nHeight) {
                throw JSONRPCError(RPC_INVALID_PARAMETER, "stop_height must be greater than start_height");
            }
        }