Duplicate content is just what it sounds like. It's when the same copy appears on two or more web pages. It can occur on your own site or copy on another site you don't control. Duplicate content is not items like footers and other content that make sense to appear on multiple pages.
Google knows this content is not the "meat" of what you are trying to say, usually based on pagination - or how your page is designed.
You Need To Check For Duplicate Content
I've found that even experienced SEO pros rarely check for duplicate content except in the beginning during Technical Discovery. This is a mistake. Duplicate content can happen when someone else scrapes your site and posts your content as their own.
It also occurs on websites because creating original content is hard, and it can be easier just to cut and paste content for similar pages. I recommend setting up a schedule to monitor for duplicate content. Some tools automatically monitor duplicate content regularly and send an alert when it is found.
Duplicate Content Monitoring
There are many different tools available to monitor for duplicate content. We use three different tools. Our first choice is Semrush.
In Semrush, the site audit report checks for duplicate content - but only on the domain. So we use a second tool to monitor duplicate content and other parts of the Web. We have found that Copyscape works best, but there are many other tools out there.
We also use Grammarly, which has a great Chrome plug-in for quick checks on sites you visit. Most of the tools are meant for teachers or others who need to check for plagiarism. These tools may not be explicitly designed for finding "duplicate content," but work great to find it.
How Much Duplication Is Ok?
As far as I know, the major search engines have not defined what exactly constitutes duplicate content. Many SEO experts have attempted to define when content goes from similar to duplicate. I prefer all content to be at least 30% different from all other copy.
I use an old "keyword density" application for this. Several tools compare two pieces of content and provide the percentage of duplication. Go to Google and query "duplicate content checker" or "keyword density tool," and you should be able to find one that works for you.