Every time I feel like writing a "pastbot" I start to think it ought to be part of the site, and then I look up top and see the "past" button! :-) Maybe it should be highlighted and blinking if it's a frequent flyer...
What do I have to gain by knowing it was posted several times before?
I'm pretty sure the community won't really like the idea of bots on HN because of spam issues and such, but what is the official policy on this? Can't find anything about bots anywhere.