[Plugin: WordPress SEO by Yoast] Robots.txt

I’m using Thesis theme. But since last month my Google Webmasters Tools was showing Duplicate Title issue due to MobilePress Plugin. I have disabled the plugin, but the issue doesn’t seems to be resolved. The numbers just kept on increasing since last month. That’s why, I have now switched to WordPress SEO plugin, which [I think] overrides the existing SEO settings by Thesis. Correct me if I’m wrong…!

The plugin offered many inbuilt features for which I was using another plugins. Like Google XML sitemaps, Facebook like thumbnail and RSS footer.

Now I have finally installed the WordPress SEO plugin, my query is that, do I need to delete the robots.txt file which I have uploaded via FTP or do I need to keep it??

Also the plugin offer inbuilt XML sitemap, so that’s why I have deleted the old sitemap [by Google XML sitemap] and added a new sitemap generated by WP SEO plugin. Will this create any duplicate content issue in search engines?? Please correct me if I’m doing anything wrong!

http://wordpress.org/extend/plugins/wordpress-seo/