You can delete any template you want once you identify where they are stored.
If you know its name you can just do a Windows search based on file name. To be
a bit more methodic, you can discover where PowerPoint stores templates in the
tutorial at http://www.soniacoleman.com/Tutorials/PowerPoint/powerpoint_templates.htm
However, be sure that it's a true duplicate that you won't ever want in the
future.
