The same content at multiple addresses.
WHERE IT COMES FROM
http and https versions both accessible www and non-www both accessible Trailing slash and non-trailing-slash versions URLs with tracking parameters Printer-friendly versions Category and tag archives showing the same posts Product variations as separate URLs The same content on a staging subdomain
WHY IT MATTERS
Ranking signals are split across versions, and engines must choose which to show. They may choose differently from you.
FIXING IT
Redirect: one canonical form, with the others redirecting to it in a single hop. This is the strongest fix.
Canonical tags: where redirecting is not possible, declare the preferred version.
noindex: for thin archive pages that serve navigation but should not rank.
THE MOST COMMON CASE
Both www and non-www resolving. Pick one, redirect the other, and set the site address in the application to match.
STAGING SITES
A staging copy indexed by Google competes with your live site. Block it with directory privacy and noindex.
CHECKING
Search for a distinctive phrase from your content in quotes. If multiple URLs from your site appear, you have duplicates.
