Occasionally I see a question marked as a duplicate of some other question, but that other question has disappeared. For example, " https://stackoverflow.com/questions/829338/online-time-logging-tool " is marked as a duplicate with links to "How do you track your hours?" and "Best tools for time tracking?", but when I click on those links, both of them give me a "Page Not Found" error.
How can a question be a duplicate of some other question, when that other question does not exist?
What should I do about such "duplicate" questions after the other question has disappeared?