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If you use the Drupal Sitemap module (also called XML Sitemap), you've probably noticed that a site upgrade will convert your search engine friendly URL aliases back to the default Drupal node URLs, i.e. /node/123, etc. This XML Sitemap problem causes URL duplication in search engines since the pathauto URLs would now be replaced by the node version.

If you are unsure or using an earlier 5.x versions, I would suggest to check your XML sitemap page to see if you have the same issue. If you find that this has happened to you, here are a few things to help correct the issue (found via Drupal). Note: I would always suggest testing on a staging site versus a live site.