What a canonical tag is
It is a line in the head of a page: a link element with rel set to canonical and an href giving the preferred address. It is a strong hint rather than a command. Search engines may choose a different version if signals conflict, such as when internal links and the sitemap point elsewhere.
When duplicates arise
Common sources are tracking parameters added to links, session identifiers, sorting and filter options, http and https versions, www and non-www versions, a page reachable under two categories, and syndicated copies of an article. Each variant can compete with the original or dilute link signals.
Where possible, fix the cause with redirects. Use canonical tags when variants must keep working for visitors.
Self-referencing canonicals
Good practice is to give every indexable page a canonical tag pointing to itself, using the full preferred address. This protects against parameter variations you did not anticipate. Most SEO plugins and builders add it automatically, so check your page source before adding a second one by hand.
Duplicate or conflicting tags on one page can cause search engines to ignore both.
Mistakes to avoid
Do not point a canonical at a page that redirects, returns an error, or is blocked or noindexed. Do not canonicalize a whole series of different pages, such as page two of a list, to page one, unless you intend to drop them. Do not use relative addresses carelessly. Avoid pointing to an unrelated page, which tells search engines to ignore the current one.
A local business with similar service pages for different areas should keep each page's own canonical, provided the content is genuinely distinct.
Checking your setup
View the page source and search for canonical. In Search Console, the URL Inspection tool shows the user-declared canonical and the one Google chose. If they differ, ask why: duplicate content, mixed signals or an unintended redirect may be at fault.
Crawling tools can list every page with its canonical and flag mismatches. Run one after a redesign or platform change.
An example with tracking links
You send an email campaign linking to your services page with extra tracking text added to the address. Clicks and shares may spread this variant around. If the page carries a canonical tag naming the clean address, search engines know to consolidate the signals there instead of treating the tracked link as a new page.
Likewise, if a product appears under two categories with two addresses, pick one as the main address, and set the canonical on the other. Keep internal links pointing to the main version so every signal agrees.
When you are unsure whether two pages should be merged, ask what a visitor would want. If both pages answer the same question, combine them and redirect. If they answer different questions, keep both and give each its own canonical.
Common questions
Is a canonical tag the same as a redirect?
No. A redirect sends visitors to a new address. A canonical leaves the duplicate visible but suggests the preferred version.
Will canonical tags stop duplicate content penalties?
Duplicate content is usually a filtering issue rather than a penalty. Canonicals help search engines pick the right version.
Do I need canonicals on a small site?
A self-referencing one is a good default, and most platforms handle it for you.
This guide is general information. Search engines change how they work and their guidelines; check Google's current documentation for your situation. Nobody can guarantee a ranking.