When it comes to writing content, you always have to write something original. In that regard, you need to focus on creating unique content. Do not just copy paste pages on your website and tweak them a bit. Try writing about different topics. This is an imperative for good SEO and positioning of your website. Even if your pages are technically ok, they won't have the necessary impact. When it comes to checking my own articles, I use Advego Plagiatus (Russian program, but it is really easy to use) and Kesor. Both of them can tell you if your article is good in comparison to other similar articles on the internet. If you have additional questions, feel free to message me directly.
Please stop this, duplicate content is harmful and search engines like Google hate it. Check this page from Google on duplicate content. Your pages should be at least 75% different unless you have some special conditions such as an ecommerce website or forums. https://support.google.com/webmasters/answer/66359?hl=en In general always go for original content even across websites let alone a single domain.