Assuming the subject line of each post conveys the subject (probably doesn't), you could add a filter that alerts any users This Post Exists and provide quick links to the post for review.
This could avoid some of the duplicate subject matter. The member could still post, but it would act similar to the notice when other posts go live before completing a reply.
Good idea to prevent duplicate topics, but unfortunately bots and spammers won't care about it, they need topics to spam to, and it's better to open a new topic and spam there instead of spamming into an existing spam megathread, because nowdays it seems that the "good old" spam megathreads have been deleted and a lot of posts of the farmed accounts has also vanished... Farmers/spammers (the serious ones)
maybe realized that it's risky to post into a spam megathread because it can disappear in a second (with the effort they put into it) so it seems they try to open new threads... And they don't care if it's a duplicate or not, unfortunately...
