...and if you have lots of errors, it sends a signal to Google that your site isn’t maintained actively and will eventually hurt your overall rankings.
This is perhaps one of the most misunderstood aspects of SEO. How Google uses signals relative to your website. So, if you have lots of duplicate title tags, Google will likely view all your title tags as not being a good representation about what that page/file is about and will likely discount the weight it puts on title tags across your entire site. If you have lots of 404 pages on your site, Google will discount the value your other signals are sending them.