You can burn all the calories you want and still eat too much or simply break even. It's also possible to take a spinning class without really pushing yourself such that you're not burning as many calories as you think.

Given that many cyclists ride 10 plus hours a week with no ill effect, I wouldn't assume that back-to-back spin classes is overdoing it. My gym offers two-hour classes for that matter. That said, from a muscle adaptation point of view, I think there's something to be said for mixing it up. Ideally, any exercise routine should incorporate restitance training, plus some core and flexibility work. But, hey, going just to spin classes is better than sitting on the couch.

I'd wouldn't concern yourself with what the regulars are or are not doing right, except the the extent it informs your own game plan. If the gym doesn't have a policy that forbids back-to-back classes, I wouldn't concern yourself with that either unless and until it impacts your ability to get into a class.