I have 2 questions. Can you use exhaust on an already exhausted champion, and if you can, does it just waste the exhaust or does it stack to increase the slow and debuffs?
Only time I can think of when you might want to cast exhaust on top of another one is if you KNOW the person that cast it before you does NOT have the summoner spell mastery for it and your enemy is about to get away/make it into tower range and he is low enough that the defense shred might make a difference between first blood and no first blood for you, or something along those lines. Otherwise chain it.
You can use exhaust on an already exhausted enemy, but it won't stack. Depending on the casting time of the first one it may last a bit longer, but I don't recommend doing that because that's a waste of spell which can turn the tides of a battle but has a long cd. So my advise is to use it wisely, and work around some tactics for example you are only using exhaust on their AD carry, your teammate uses it on AP caster etc.