Spammers badly need spam-free content so they can mix some legitimate links with the junk they spew.
One great Black Hat SEO trick is to find where your competitors are getting clean links and insert your own links there so they do your spamming for you.
If I have (say) 15 affiliate marketing sites, I might make a link aggregator site that looks a bit like Hacker News. Except I won't make just one, I might make 30 of them.
These might subscribe to a bunch of RSS feeds and randomly select articles, maybe 10% of the links on those sites go to my affiliate sites.
If you can inject spam into those RSS feeds that system I describe would amplify it and this could have effects ranging from: you are using my marketing machine to promote your content to my sites getting really obnoxious and getting blocked.
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"Duplicate Detection" is a necessary technology for web search because s̶h̶e̶e̶people copy themselves and other people without bound. It cuts both ways because Google and Bing have no sure way to know which one is the copy and which is the original. So (1) they aren't completely efficient at removing duplicates and (2) duplicate detection can be turned into a weapon against you, just like that link aggregator.
One great Black Hat SEO trick is to find where your competitors are getting clean links and insert your own links there so they do your spamming for you.