What about canonical URLs and duplicate content?
With a canonical URL you suggest to a crawler the most relevant URL of a resource with
more than one URL, for example “about-us” and “about-us/”. Without a canonical URL a
crawler may decide you have duplicate pages just to get higher ranking, and will penalise
you with lower ranking.
Let’s say you have one page in English and one page in Swedish, but you want a film resource
to be shown for both languages. You can use a canonical URL to tell a search engine that,
even though you have duplicate content (in this case the film), the film is relevant in both
pages. Using a canonical URL for the film will not risk lower your search engine ranking.
When using a canonical URL you suggest to a crawler that this is the most relevant URL for a
resource. When using a redirect (301) you are directing a visitor to a specific page.
However, a crawler can ignore canonical URLs if they don’t match reality, so they must be correct.