There is already the duplicate question filter when a user asks a question, and the "related questions" area, so duplicate questions should be easy to spot.
However, the dupe checked could use a bit of work IMO, often it returns completely unrelated questions. Maybe it should simply not show questions below a certain threshold, it seems like if a question has all unique words apart from "the" then it will pick 5 questions with "the" in the title instead of no questions.