SEO
Is duplicate content a penalty?
No, not in ordinary cases. There is no duplicate content penalty. The cost is that ranking signals get split across URLs and the engine picks a canonical version that may not be yours — which is a competition problem rather than a punishment.
Internal duplication
- Trailing-slash and non-slash variants both returning 200
- HTTP and HTTPS, or www and non-www, both resolving
- Tracking parameters creating unique URLs
- One product reachable through several category paths
External duplication
Publishing the manufacturer description that two hundred other stores also published is not a penalty. It gives the engine no reason to prefer your page, so it prefers whoever has more authority — which is usually not you.
The fix
Canonicalise deliberately, redirect variants in one hop, and rewrite descriptions on the products that actually sell. Twenty rewritten properly beats two hundred rewritten badly.
How to find it on your own site
- Load your home page four ways — with and without www, with and without a trailing slash — and check each returns one canonical URL rather than a 200
- Add a tracking parameter to any URL and confirm the canonical still points at the clean version
- Search a distinctive sentence from a product page in quotes to see who else published it
- Compare your indexed page count against the pages you deliberately publish
The longer version
This answer is deliberately short. If you want the full treatment, Technical SEO, in the order the problems actually block you covers it in about 4 minutes — free, ungated, written from client work.
Who answered this
Anas Bin Masud builds e-commerce sites and does technical SEO for businesses in the UK, Canada and Pakistan — fifteen live client sites, six of them stores taking real payments. These answers come from those projects rather than from a content brief. More about how I work, or ask me something directly.