As a follow up to CVE-2018-17144, this removes the unused `fCheckDuplicateInputs` parameter and explains why the test can not be disabled. Apart from protecting against a dumb accident in the future, this should document the logic in the code. There is a technical write-up that explains how the underlying coins database behaves if this test is skipped: https://bitcoincore.org/en/2018/09/20/notice/#technical-details. However, it does not explicitly mention why the test can not be skipped. I hope my code comment does that.

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