Why should I avoid duplicate pages?

Duplicates are pages with nearly or completely identical text on the same site at different URLs. Yandex’s example of one home page shows how ordinary this is:

• https://example.com/
• https://www.example.com/
• https://example.com/index
• https://example.com/index.html
• https://example.com/?utm_source=…&utm_medium=…&utm_campaign=…

The robot groups them and picks one for search — and may change its choice later, which is why an indexed URL can shift without anyone touching the site.

The four costs

• Several pages are indexed instead of one, wasting the site’s resources and Yandex’s.
• New pages take longer to be indexed.
• Duplicates compete with each other in results.
• An important landing page can be judged the duplicate and excluded in favour of a lesser version of itself.

That last one is the reason this matters commercially. The page you invested in is not guaranteed to be the survivor.

Where they come from

Chiefly the content management system: trailing-slash variants, index-file addresses, printer-friendly versions, and every campaign parameter appended to a shared link. None of these are authored deliberately; they accumulate.

The remedies, matched to the cause

• Address variants (www, protocol, /index.html) — 301, and a move request where it is a mirror.
• Trailing slash — 301, not canonical, which Yandex says may be ignored here.
• Campaign and tracking parameters — Clean-param, which also transfers accumulated metrics rather than discarding them.
• Genuinely similar content pages — rel="canonical", or consolidation into one better page.

How to see the scale

Searchable pages → Excluded pages, filtered to the duplicate status, or the downloadable archive where they carry DUPLICATE. Group by pattern: the list is nearly always a handful of causes multiplied.