Prevent objects from registering the same observer more than once.

This could lead to unintended side effects and difficult to track down bugs.

We ran into this problem on my team and were under the assumption that this wouldn't happen. Digging into the source code of `ESCObservable` I realized that you can have duplicate observers for the same object. This seems like a bug and not a feature (at least by our assessment), so I whipped up a quick fix to prevent that.