Why to avoid duplicate content
Believe me when I say duplicate content is probably the most hurtful thing you can have on your website. There are 2 main types of duplicate content, site wide - which refers to the same content across pages on your site and across sites - which refers to the same content across different websites.
Duplicate website content
The problem with duplicate content across your own site is that it creates confusion within the eyes of a search engine. Obviously things like a navigation menu, headings etc. will be duplicated across most or all pages of your site but in general you should rewrite everything differently even if you have the same or similar topics across pages. One things like e-commerce sites where you can have different products across different pages more or less the exact same only with a slight difference, let’s say colour, there’s not a huge amount of variation you can make with the page content. In these circumstances it doesn’t really matter, for something like a product page you will primarily target probably a single keyword, things like the product model number, which should be done through the Meta and the page content only really relates to the user as it will probably detail the product specs in a general way. But it is different for important landing pages that target a large number of keywords that act as gateways to different sections and pages within your site. When you break up your site into different pages / categories you want unique content across all of these pages detailing exactly what the page is about and depending on the pages underneath maybe giving some detail about those also. If you have two landing pages that target two different keywords but have a single block of text that is the exact same on both, you waste the opportunity to tailor that pages content to its specific keyword and as stated before - you present search engines with the question, these pages are almost the exact same, which do I pick?
Duplicate page content
Duplicate content across sites shouldn’t be an issue as long as you are not the one copying content from another site. You will find that if you have a lot of unique written content on your site people will just copy and paste it onto their own site, sometimes linking back to you and giving you credit, sometimes passing it off as their own. It isn’t too much of an issue if someone decides to copy your content as long as you had it indexed first. Never copy content from another site in an attempt to pass it off as your own. You don’t get rewarded for stealing other peoples work. Of course there will be situations where you may genuinely need to include other peoples content either for analysis or example, in this case link back to them and make sure to emphasise that the work belongs to the author. Duplicate content across domains will always be a huge problem, but let it be a problem for other people, it’s not something you should really worry about if you generate you own content.