Is there any accepted timeframe for duplicates?
Currently, no. It seems worthwhile to keep old quotes visible, but I suspect that would be better accomplished by automatically generating a database of rationality quotes from these threads (like DanielVarga's best of collections), and then displaying a random one on each LW page with frequency related to the number of upvotes they received, say. I don't think that duplicating quotes in quote threads is a good idea, because this focuses effort on finding new quotes and material to incorporate into a growing body of knowledge rather than rehashing previously found knowledge.
I endorse (with the possibly-expected caveat about Wilson score ranking).
Unfortunately, I can't (don't know how to?) hack the LW backend. Is that something I can look into?
Another month has passed and here is a new rationality quotes thread. The usual rules are: