Duplication of content, titles and meta descriptions hurt SEO in one way or another. Search Engines encourage use of unique content, title tags and meta descriptions. Search engines have a filter in place for duplication of content. So, if two pages have similar content, one may lose its power to rank organically in search.
Duplication of Content:
One of the most common on-page SEO issues many sites face is duplication of content within and across domains.
Google filters results for duplication of content. If a site has duplicated content, it confuses Google which page to show in search results for a keyword.
Too much of duplication within a site may result into a Google penalty. Small extent of duplication can be ignored by Google.
You can check for duplicated content using different tools including Copyscape.
Duplication of Title Tags
Title tags are one of the most important on-page ranking factors and they tell search engines about the topic of a page. Duplicated page titles waste some valuable space from SEO perspective. Few sites have same title for all the pages, which is a clear SEO mistake.
You can check for duplicated title tags using different tools including Google Search Console and ScreamingFrog.
Duplicated Versions of the Home Page
Another common duplication to avoid as far as on-page SEO is concerned is duplicated version of the Home page (or any other page). Few sites have different version of the Home page all accessible to the search engines.
Having duplicated versions of the home page may result into loosing valuable internal and external link value.
This problem can be fixed by redirecting (301 redirects) all version of the home page to one main official version of the Home page.
Duplication of Meta Descriptions
Meta descriptions are like your ad in search results. So, meta description should be unique and be conveying what the page is about, to the audience.
If you cannot come up with the original meta descriptions for some reason, Google recommend leaving the meta description blank instead of duplicating them.
You can check for duplicated meta description using different tools including Screaming Frog.
Duplication is unavoidable in some cases, but proportion of duplication should not be much for a domain in any case. You can use different tools and settings to find and avoid duplication of content. Above all, writing original and compelling on-page content takes you long way forward in online world.