Opening two of the same wallet (tested in v0.18.0 of bitcoin-qt 64 bit) will cause a database collision, and will crash the program.

`[marshall@likon] {21:21} [~] $ bitcoin-0.18.0/bin/bitcoin-qt `
`terminate called after throwing an instance of 'std::runtime_error'  what():  BerkeleyBatch: Can't open database 8204388-wallet.dat (duplicates fileid 8b1c090000000800e25686350000000000000000 from 1352491-wallet.dat)`
`Aborted (core dumped)`

I have been able to reproduce the issue by importing two of the same wallet (with different filenames).
{
    std::map<CTxDestination, CAmount> balances;
    {
        LOCK(cs_wallet);
        std::set<uint256> trusted_parents;
        for (const auto& walletEntry : mapWallet)
        {
            const CWalletTx& wtx = walletEntry.second;
            if (!wtx.IsTrusted(locked_chain, trusted_parents))
                continue;
            if (wtx.IsImmatureCoinBase())
                continue;
            int nDepth = wtx.GetDepthInMainChain();
            if (nDepth < (wtx.IsFromMe(ISMINE_ALL) ? 0 : 1))
                continue;
            for (unsigned int i = 0; i < wtx.tx->vout.size(); i++)
            {
                CTxDestination addr;
                if (!IsMine(wtx.tx->vout[i]))
                    continue;
                if(!ExtractDestination(wtx.tx->vout[i].scriptPubKey, addr))
                    continue;
                CAmount n = IsSpent(walletEntry.first, i) ? 0 : wtx.tx->vout[i].nValue;
                if (!balances.count(addr))
                    balances[addr] = 0;