Someone shared with me their feed where the GUID problem exists. Unfortunately, this value is being pulled from the WordPress database and isn't something the RSS Includes Pages plugin creates or modifies. So if the value is blank in the database (which it appears to be hence the error about duplicates) then your feed won't validate.
If you're familiar with MySQL, you can go into your WordPress database and add a unique value to the GUID field in the WP_POSTS table. Of course, if you have a lot of pages this isn't a particularly good solution.
There was a plugin that would've corrected this issue (http://wordpress.org/extend/plugins/guid-fix/), but it looks like the author is no longer supporting it and has indicated that his solution is no longer the correct one. I'm going to contact him to see if he knows of another fix. Might help if he heard from others too!