Payday 2 "Instance already running" issue

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I added Payday 2 as a game to the program, started it up, then closed it.

Upon attempting to start Payday 2 up again, it tells me there's an instance 
already running, when there clearly isn't. I've even checked Task Manager, it's 
not running.

Version: Alpha (v2, r49) on Windows 7

Original issue reported on code.google.com by 404UserNotFoundTF2 on 21 Aug 2013 at 5:49