What Are On-page SEO Mistakes?
Duplicate content is a significant problem. There's a penalty named Google Panda that penalizes sites with content quality problems. Among the causes could be poor or scraped web page copy, thin content, duplicate titles, meta descriptions, headings. I've handled cases like the one when I was working a landing page with the owner having built unbeknownst to me 2,000+ pages with similar content, titles, descriptions to my target page.
Duplicate content is an SEO trap; throttles organic performance; reduces page indexation; demotes the duplicate pages or the whole site if it's a widespread problem like the one above. Canonical tags and backlinks can't save the day. I was creating content, building backlinks only to discover an enemy within. The fix is a technical SEO audit, difficult and costly. The owner wouldn't consider spending money on doing a professional job, so he tried to correct things by himself. I gave him a couple of guidelines, but the audit needs to be thorough and run with an expert eye. Needless to say, after weeks of him changing pages there was no progress.