Even if there isn’t a “penalty” for having duplicate content on your website, it can nonetheless negatively impact your organic traffic because of the following factors, not because Google devalues your site:

Because you’re competing with both your competitors and yourself, search engines are unable to determine which page should be ranked first.

Two or more pages may end up vying for rank as a result of this confusion. Occasionally, a query may function rather well across numerous pages. But in most circumstances, depending on the website’s authority in the subject, for instance, both sites will perform poorly or not at all. A single page with superior quality will be able to carry greater weight.

You can get backlinks to both pages.
Backlinks are, to put it simply, links coming from other websites that bear the seal of approval. By transferring link equity and traffic from the original URL, they can benefit a URL. 

The likelihood is that the less authority page will begin to compete with the authoritative page if you have 100% identical duplicate content and it receives backlinks. This will cause the two pages to share equity. As a result, both pages function at the same level, but only one page would enable greater equity to be developed.

The efficiency of your crawl budget is low.
The quantity of pages Google visits on your website within a given time frame is known as the crawl budget. This figure fluctuates by site and day to day. However, having pointless pages on your website prevents Google from effectively crawling your most crucial sites. This increases Google’s workload and may prevent your new and/or authoritative pages from being efficiently crawled, indexed, and ranked.

Now that you know what duplicate content is and how detrimental it is to your website, you may be wondering how to find duplicate content on your website. Depending on the magnitude of your website, it might frequently seem like an overwhelming task.