Strange behavior. The TD launcher tries to prevent 2 instances at once, but I can envision a scenario where it won't be successful. However, the HTML application host inside of which it runs is also supposed to prevent it. I have seen this happen when the host gets in a weird state. Usually the problem becomes you can't launch a second instance, because there's still one running (but without a window), which leads the user to think the software is broken. Solution is to kill the rogue instance and start it up again, or reboot for the tech-challenged.
Thanks for reporting this. Not much you can do, just kill the process. I doubt it's hurting anything, either.