Large Number of Pages Pointing to the Same Canonical URL
This message alerts you to the fact that there appears to be an issue with the use of canonical tags on your site: Bing has detected that many URLs that contain substantially different page content are pointing to the same canonical URL in the canonical tag.
The alert looks something like this:
Large number of pages pointing to the same canonical URL
Canonical tags can be used to help de-dupe URLs for pages with the same or very similar content. Bing has detected that your site is using what appears to be the same canonical URL for a large number of pages that are not the same. This may be indicative of a problem with your HTML template, content publishing system, or site code. You should review the source of your pages and check if the URLs used inside the <link rel="canonical" href="<url>"/> are not accidentally pointing to the same location for all (or too many) pages. Learn more.
The Bing Webmaster Team
When you have received this alert, you should check how your site implements canonical tags and make sure that unique pages point to unique canonical URLs. Issues with your site's implementation of the canonical can lead to ranking issues and may cause us to ignore canonical tags from your site altogether. For example, you should check your web templates for boilerplate HTML or code that may accidentally re-use the same URL as the URL that is used in the canonical link tag. On thing to check is that they don't accidentally all point to the homepage URL of your site.
Canonical tags are correct, but I am still getting the alert
If you have verified correctness of the canonical tag but are still receiving the alert, the alert may have been triggered due the fact that you have a large number of URL variations that are caused by URL parameters. For example, your homepage may be accessed and linked-to using tracking code parameters with many different variations in their values. Although the canonical tag prevents these variants to be indexed, the preferred way to deal with URL parameters is to use the Ignore URL Parameters feature to normalize these URLs first.