And informing people of a useful site like KarmaDecay would help reduce reposting, right?

Yes, but why would we want to reduce reposting? That's the whole question here. In fact, reposts are valuable because reddit is dynamic. Not everyone spends every day on reddit, people miss posts, many new users join, old users return, reddit algorithms change, upvote ratios and opportunities for front-paging change, interests change, and on and on and on. A repost is effectively the same as a post for a great majority of redditors. There's no difference if they, personally, haven't seen it before.

It's a delicate balance, and I'd perhaps downvote if it were reposted from last week, but in general the very idea of reposts is not to be decried, but supported. It comes from the realization that you are not the only person on reddit, that it is a community full of different ages, people, working habits, time zones, and internet abilities, and reposting content that hasn't been seen in a long time is beneficial to that community. It brings it together in ensuring a better understanding of the type of content the typical redditor enjoys, especially if it's highly upvoted like this post.

I honestly can't tell if you're being serious here or not.

likewise

I assure you he's being very serious. There's a damn good reason "don't complain about reposts" is the number one rule of the reddiquette.

A reddit bereft of reposts would be a sad, boring place. If you want better content on reddit, post better content. Encourage it by upvoting. Follow the reddiquette and support posts which you like, and downvote repeats if you don't think they should be seen by anyone ever again (I find this idea ludicrous, but to each his own). A repost filter would actually reduce the quality of content on reddit, because there would simply be less quality content at any given time; the reposts themselves are mostly harmless and debatably beneficial. They should be praised, upvoted if good, and downvoted at the very worst. Anything more than that is a detriment to the community.