You have the Articles linked to two separate categories…how…hmm…I’m not sure as Joomla doesn’t do this natively. So it must be an addon you are using that is causing the problem.
My first place to look would be your “Commenting” Components/Modules/Plugins like Shoutbox or others that use “jc_comment”
If you are using a CCK (K2, Zoo, etc.)- Content Construction Kit – that may be the issue as well.
To test, simply one by one disable the Component/modules/plugin until you find the offending addon. Once found, it should be easy enough to diagnose how to make it stop the duplication…or whether it can’t help itself and needs to be put down.
Have fun.
John.