Fixed Same page opening with 2 different URLS - Google Webmaster Issue

One is a friendly URL and one is not. Google must have indexed the first link before you enabled friendly URLs, and now both work even though your forum is currently only using the second link. XenForo addresses this problem for important routes by enforcing the canonical URL, but it does not do this for recent-activity or many other less important routes.

I will mark this as confirmed. The devs may wish to canonicalize more URLs.
 
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