Without a doubt, you can perform too much exercise. If you want your body to get anywhere, you need to allow it to recover. Even top athletes cannot exercise all-day every-day, otherwise their bodies would simply shut down.

Recovery, recovery, recovery - it is so VITALLY important. More is not necessarily better.

Of course, your goals will dictate the magnitude of exercise that is required. However, performing excessive exercise will work against you.

Kek once mentioned that "muscle is more expensive to maintain than fat" - which is a very good way of considering muscle versus fat retention.  Muscle requires a significant amount of energy to maintain. You do too much exercise and/or don't eat enough - your body will turn on it's survival mechanisms and prefer to rid itself of the muscle mass rather than the fat mass.

But it's not only muscle - if you perform too much exercise, your body will begin to do things to tell you to SLOW DOWN. Hormones begin to be secreted that halt progress in an attempt to survive and recover. You will feel tired and lethargic - hindering the intensity of your workouts. You can become moody and lose sleep. Furthermore, because your body can become so run down - this will encourage infection because your immune system will ultimately become very weak.

Performing too much exercise can lead to overtraining which basically places your body in "survival" mode. Whilst overtraining is a vague term that covers a lot of situations, when you overtrain, your body stops progressing (or plateau) and your level of performance will decline significantly.

Personally speaking, I used to think that "more is better". I employed this way of thinking when I first began my training regime (we are talking years ago now) and in a year I made next to no progress (it's so easy to live in denial).

You also need to learn to listen to your body. There is no formula to determine the amount of exercise required. Your body is the best indicator of whether or not you are overdoing it. Feeling excessively run down is your bodies way of telling you to slow down. It can be hard for many of us to "skip" a gym session - but this discipline can be what separates good results from GREAT results.

Unfortunately the importance of recovery is overlooked. Here's a good article that provides an overview on the topic:

Recovery 101: Creating the Ultimate Physique

And when you finish with that, there's a tonne of other articles on the website dealing with this VERY important issue :)