The Problem: You run a website that allows for user-generated content like job listings, rental properties, classified ads, personals or even UGC products (think eBay or Etsy). Thoughtless users, thinking only of themselves and the time they’re going to save, wreck your SEO by posting the same content they’ve put up on ten of your competitor’s sites. This creates duplicate content issues for you instead of that dream of Web 2.0 SEOs – free, unique content. It can definitely be frustrating.
The Symptoms: The search engines start by simply not listing your pages, but sometimes it gets more severe and whole subsections of your domain go unspidered or unindexed because the engines’ algos have determined that you’re a clearing house for material they’ve already seen and don’t need again. In rare cases, this might even cause completely unique, valuable material to be excluded from the visible SERPs.