01
Why does a contractor's site end up with duplicate URLs?
Usually by accident, not by copying. Google's canonicalization documentation lists common causes, including HTTP and HTTPS versions of the same site, separate mobile and desktop versions, sorting and filtering features, and accidental variants such as a demo copy of a site left open to crawlers. Any of these can give one page two or more addresses.
On a service business site, the usual versions look like this. Ad and email tracking codes add text to the end of a service page's address. A web designer's staging copy stays open after launch. The www and non-www versions both load. Each address shows the same page, and Google has to pick one. For www, non-www and HTTP versions, Google's advice is to pick one address and redirect the others to it. For tracking codes, a canonical tag pointing to the clean address states your choice, and a staging copy belongs behind a password. Many website platforms and SEO plugins can add canonical tags automatically, so check where yours point.
02
Does Google always follow a canonical tag?
No. Google says it "may choose a different page as canonical than you do, for various reasons." It weighs the tag alongside other signals. Its documentation calls a redirect and a rel="canonical" tag "A strong signal" and listing a URL in your sitemap "A weak signal," so when those signals disagree, Google's pick is harder to predict.
Duplicate URLs aren't a penalty. Google says "Some duplicate content on a site is normal and it's not a violation of Google's spam policies," and its SEO Starter Guide adds: "If you have some content that's accessible under multiple URLs, it's fine; don't fret about it." The practical cost is smaller and more annoying: Google may show an address you didn't choose, such as one with tracking code on the end.
03
When should you use a redirect instead?
When the old URL should stop existing. If you rename a service page, merge two pages or move to a new domain, a permanent redirect sends visitors and search engines to the new address. A canonical tag fits when both URLs need to keep working, such as a page that's reached both with and without an ad tracking code.
Don't reach for robots.txt here. Google says "Don't use the robots.txt file for canonicalization purposes," and warns that "Google may still index URLs that are disallowed in robots.txt without their content." Blocking a duplicate hides the page from Google's crawler without telling Google which version you prefer. Our guide to website migration SEO covers redirect mapping when a whole site changes, and the robots.txt entry explains what that file does control.
04
Can a canonical tag fix near-identical town pages?
It can hide them, but it doesn't fix them. Pointing 30 town pages at one main page tells Google you consider them duplicates, so it can choose to show only the main page. The near-identical pages are still on your site. If they exist only to rank, they may also fit Google's description of a doorway page.
There are two honest choices. Make each town page useful on its own, with local jobs, local questions and details a customer in that town would want. Or merge them into one service-area page and redirect the rest. Producing pages in bulk with little to add is the pattern behind scaled content abuse, and no tag changes that.
Related terms: robots.txt · Doorway page · Schema markup · All glossary terms
Where to go next: Our technical SEO checklist shows where canonical tags fit among the other checks a contractor's site needs.