How should you handle duplicate content and canonical URLs during a website migration?

During a website migration, first identify which old and new URLs serve the same content, then choose the canonical URLs for the migrated site. When content moves permanently to the new site and the old URLs no longer need to serve separate pages, prioritize 301 redirects from each old URL to its corresponding new URL. If both sites must retain the same content, you can use canonical tags to specify the preferred version. The key is to ensure that redirects, canonical tags, and the sitemap send consistent signals—not to assume that adding a single tag completes the migration.

Identify duplicate pages first, then decide which URL to keep

Google Search Central explains that Google automatically detects duplicate content and assigns a canonical page to each group of duplicate pages. Proactively identifying duplicate pages for Google can also help prevent excessive crawling. The issue to address during a migration is therefore which URL should represent a given piece of content; there is no need to treat the coexistence of the old and new sites as a plagiarism issue from the outset.

Before implementation, prepare a mapping list that distinguishes the following situations:

• Content is moving permanently: list each old URL and the new URL that will host its content.
• Both sites must continue publishing the same content: first decide which version will be canonical.
• The migration also involves a change between HTTP and HTTPS or a domain change: check whether the different versions still serve the same pages so that no duplicate access points are overlooked.

Use redirects for permanent moves; consider canonical tags when both versions must remain

A relevant answer on Webmasters Stack Exchange recommends redirecting old URLs to new ones where possible, so that users and Google know the content has moved permanently. If the content must be published on both sites, it suggests pointing canonical tags to the primary site. This is community advice, not an official Google migration rule.

For example, if an article moves from an old domain to a new one and the old site no longer needs to display it, the approach is to redirect the old article URL to the new article URL. If both sites still need to display the same article, that is the two-version situation discussed in the answer: choose the primary article URL and point the other version’s canonical tag to it.

awoo’s 2025 event recap also recommends coordinating 301 redirects, canonical tags, the sitemap, and the correct HTTPS status to provide consistent canonicalization signals. Based on this advice, check whether the redirect destination and canonical tag point to the same preferred version, and whether the sitemap is consistent with that choice.

Canonical URL changes and full-site migrations have different timelines

Do not assume that search engines have completed the transition just because the new site is accessible. In an October 2026 report, Search Engine Journal relayed timelines presented at a Search Central Live session: canonical URL changes typically take around one to three weeks, potentially extending to several months when signals conflict; website migrations typically take around one to three months, with slower cases taking six months to a year or more.

These are reference ranges compiled in the report, not completion deadlines for every website, nor do they guarantee when rankings or traffic will recover. If old URLs still appear after the migration, first check whether the old and new versions send consistent signals rather than judging success or failure solely by how many days have passed.