Well, since I only reindex once a day, it wouldn't really help the current duplicates issue. Those that we've had dupes on, have been because of very hot stories everyone wanted to share.
I am not a fan of the on-the-page search box. The places I've seen them used, they get abused and most often lead to frustration by users. Some of the highest levels of site abandonment that I've experiences, have been at the hands of search boxes.
The best thing we can do for dupe topics, is use good titles so that people can spot the story from that forums index page.