The structure of a website influences its crawlability, or how easy it is for a search engine’s bots to crawl a site and determine what it’s all about. Users (and bots) should be able to navigate from page to page with ease, and should be able to reach any given page in as few clicks as possible. In other words, there should be a clear pathway between links.
Duplicate content is a very important issue. As the name implies, this happens when there are more pages on the site with the same or similar content under different URLs. It’s a common mistake on sites that have a lot of content. This is very bad for the search engines as they have a hard time deciding which page to include in their indices. As a result they use algorithms to penalize duplicate content which then leads to lower visibility for each page and consequently lowering the site’s rankings. The issue can be resolved by using the canonical URL tag in the page header.