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).
{
    for (const std::shared_ptr<CWallet>& pwallet : GetWallets()) {
        pwallet->Flush(false);
    }
}
void StopWallets()
{
    for (const std::shared_ptr<CWallet>& pwallet : GetWallets()) {
        pwallet->Flush(true);
    }
}
void UnloadWallets()
{
    auto wallets = GetWallets();
    while (!wallets.empty()) {
        auto wallet = wallets.back();
        wallets.pop_back();
        RemoveWallet(wallet);
        UnloadWallet(std::move(wallet));
    }
}