You can pick up on such content manually from various sections of your site, but the quickest way would be using tools such as Screaming Frog, Copyscape and SEMrush. These are instruments that surface the exact (or quite similar) duplicated sections of your site. Periodic checks are necessary, as even the tiniest differences in product descriptions, blog articles or meta tags can produce duplication problems.

Yes, it can. Although Google technically doesn’t “penalize” for duplicate content, it can still suppress your pages because they don’t rank as highly. Google attempts to select the most relevant version to display in search results, so some pages may not show up at all. By eliminating duplication, you are getting the best possible SEO mileage out of your content.

Common causes include:

Several URL variants of the same page (with or without a tailing slash, in www and non-www version)
Print-friendly or Mobile versions of articles
Content scraped or copied from other sites.
Identical product descriptions on multiple pages
Session IDs or query string parameters in URLs

It helps avoid similar trouble in the future, if looking to fix current problems.

Some solutions for duplication are there according to the way they have been duplicated:

301 redirects: Redirect duplicate URLs to the most desired page.
Canonical tags: Inform search engines which page is the “original” if you have multiple pages with essentially the same content.
Noindex equivalent: Don’t let low-value duplicates get in the index.
Redraft or condense content: Rewrite your duplicate pages, make them unique or thin the herd.

The trick is to select a solution that makes users happy while properly guiding search engines.

A canonical tag (aka "rel=canonical") is a way of telling search engines that a specific URL represents the master copy of a page. If, for example your product can be found at multiple URLs then you want a canonical so Google knows to assign that linking power to one page only and not get it diluted.

Absolutely! As soon as the search engines know what original and authoritative pages are, your primary pages cull more favourable rankings. This gives more visibility, clicks and traffic. It’s also better for users too as they will be referred to correct content instead of having numerous options and things being extremely confusing!

Conduct these checks, if you can, every quarter—or at least when you add substantial content. High-traffic e-commerce sites or blogs might need monthly checkups. Keeping track lets you snag duplication by the throat early on, and can stop packaged SEO problems from accumulating unchecked across your site.

Yes. Internal linking throughout will assist search engines in recognizing your most important pages. Directly linking to the canonical of a page enhances its authority. However, associating with differing duplicates can also make matters worse by showing search engines mixed signals.

It’s ongoing. Although not having additional copies of content may fix this problem in the short term, each time you add products/blogs/mirrored pages, new duplicate content can reappear. Implementing a proactive strategy to these—by way of canonical tags, redirects and unique content creation—will help to keep your site SEO friendly for longer.