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)
filter = filter | ISMINE_WATCH_ONLY;
    }
    bool include_removed = (request.params[3].isNull() || request.params[3].get_bool());
    int depth = pindex ? (1 + chainActive.Height() - pindex->nHeight) : -1;
    UniValue transactions(UniValue::VARR);
    for (const std::pair<const uint256, CWalletTx>& pairWtx : pwallet->mapWallet) {
        CWalletTx tx = pairWtx.second;
        if (depth == -1 || tx.GetDepthInMainChain(*locked_chain) < depth) {
            ListTransactions(*locked_chain, pwallet, tx, 0, true, transactions, filter, nullptr );
        }
    }
    
    
    UniValue removed(UniValue::VARR);
    while (include_removed && paltindex && paltindex != pindex) {
        CBlock block;
        if (!ReadBlockFromDisk(block, paltindex, Params().GetConsensus())) {
            throw JSONRPCError(RPC_INTERNAL_ERROR, "Can't read block from disk");
        }
        for (const CTransactionRef& tx : block.vtx) {
            auto it = pwallet->mapWallet.find(tx->GetHash());
            if (it != pwallet->mapWallet.end()) {
                
                
                ListTransactions(*locked_chain, pwallet, it->second, -100000000, true, removed, filter, nullptr );
            }
        }
        paltindex = paltindex->pprev;
    }
    CBlockIndex *pblockLast = chainActive[chainActive.Height() + 1 - target_confirms];