What duplicate content actually costs you
When two or more URLs show Google almost the same content, Google has to guess which one to rank. It splits the links, the authority and the relevance signals between them, then ranks whichever it picked, usually not the one you'd have chosen, and usually lower than a single consolidated page.
It also wastes crawl budget. Google spends its time re-reading duplicate versions of pages it already knows instead of finding your new or updated ones. On a big catalogue, that matters.
The big Shopify culprit: /products/ vs /collections/.../products/
By default, Shopify makes the same product reachable at two URLs: the clean one (yourstore.com/products/x) and a version nested under whichever collection you clicked through (yourstore.com/collections/y/products/x). Same product, same content, multiple addresses.
The good news: modern Shopify themes usually set a canonical tag pointing the nested versions back to the clean /products/ URL, which is exactly the fix. Check your theme is doing it, view source on a product reached via a collection and confirm the canonical points to the bare /products/ link. If it does, this one's handled.
Filters and sorting: the near-infinite duplicate machine
Faceted navigation, filtering a collection by colour, size, price, and sorting it by 'best selling' or 'price low to high', generates a fresh URL for every combination. Each one shows a reshuffled slice of the same products. That's potentially hundreds of thin, near-duplicate pages off a single collection.
Left unchecked, these get crawled and sometimes indexed, bloating your index with junk that competes with the real collection page. Make sure filtered and sorted URLs canonical back to the clean collection, or are blocked from indexing, so your one real page keeps all the signal.
Tag collections you didn't know you had
Shopify auto-creates collection pages from product tags (yourstore.com/collections/all/tagname). If you tag heavily, you can end up with dozens of automatic, overlapping collection pages nobody designed, all thin, all competing with your real collections.
Keep a tight, intentional set of collections. Stop tag-based and /collections/all filtered URLs from being indexed so they don't dilute the pages you actually built.
The quieter ones: www, trailing slashes, and pagination
A few smaller sources add up. Your store resolving on both www and non-www, or with and without a trailing slash, can create duplicate versions, pick one and redirect the rest. Paginated collection pages (page 2, 3, 4) shouldn't be treated as duplicates of page 1, but they shouldn't compete with it either; canonical each page to itself, not back to page 1.
None of these are dramatic on their own. Together they're the difference between clean signals and muddy ones.
How to check and what to do
Do a site:yourstore.com search and look for the same product or collection appearing under multiple URLs, that's your duplication showing. Then view source on your key pages and check the canonical tag points where you want the ranking to land.
In order: confirm your theme canonicals nested product URLs to the clean /products/ version, stop filtered/sorted/tag URLs being indexed, fix www and trailing-slash consistency with redirects, and leave pagination self-canonicalling. Do that and Google stops guessing, and concentrates every signal on the one page you want to rank.
More on getting indexed and found on Google: Shopify 'Crawled - currently not indexed': what it means and how to fix it · Why isn't my Shopify store showing up on Google? · Why your Shopify store is slow, and what actually speeds it up · Why did my Shopify traffic suddenly drop? · Technical SEO checklist.