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src/bitcoind -daemon -regtest
src/bitcoin-cli -regtest stop
cp ~/.bitcoin/wallet/wallet.dat ~/.bitcoin/wallet/wallet2.dat
src/bitcoind -daemon -regtest
src/bitcoin-cli -regtest loadwallet wallet2.dat # The first time it would fail to load wallet
src/bitcoin-cli -regtest loadwallet wallet2.dat # The second time it would succeed to load wallet, but this would cause error because of duplicate fileid
It would flush wallet.dat and close Db* in mapDb['wallet.dat'].second, so mapDb['wallet.dat'].second would be nullptr. Then there is no available Db* to detect if the wallet fileid is duplicated.
I'm not sure if the issue is related to #14163, but I found this while debugging it.