Fatal error: Cannot declare class Single_Post_Template_Plugin

Hello,

I recently moved to a new host and everything was working great until I decided to update the site with new information. I added a Portfolio item and it seems it created just fine and all images and so on appear properly in thumbnails on other parts of the site.. but when I try to access that new portfolio page, I get the following error:

Fatal error: Cannot declare class Single_Post_Template_Plugin, because the name is already in use in /var/www/example.com/public_html/wp-content/themes/oblivion/post_templates.php on line 2.

Both wordpress and the theme are using their latest versions. Any idea why that may be happening?

Thank you.