Interesting. Well, thanks for providing access to your server... I wouldn't have been able to figure this one out without it.
It appears that if you're quick enough with the mouse, and maybe not so familiar with web pages being "single-click" interfaces rather than "double-click interfaces", it's actually possible to double-click on the "start autoDJ" button and start two copies of it.
I would have thought the second request would be delayed long enough by PHP's session management code for the first request to establish its internal lock, but... apparently not.
Will have this fixed for the next build.