When you exhaust someone who is already exhausted, it refreshes the timer. You can think of it as the first is overridden I suppose. BUT if the first person to cast it has summoner mastery for it and the second person does not (and vice versa I suppose as well) when will the improved armor and magic resist be applied in these situations?
Exhaust is not worth using twice on a single target. However, applying a second exhaust without the summoner mastery will not affect the first one in terms of the improved armor and magic resist.
I haven't checked this but I would assume that if the first person had the mastery and the second person did not have the mastery, when the second person used Exhaust it would override the first person's exhaust and the target would only have normal Exhaust on them.