I spend too much time on SO.
One result of that is that it's fairly easy for me to see questions that are duplicates of one posted a few days before - I still remember seeing that one. About half the time, I can find the duplicate quickly in a search.
Such a question should get a close vote from me with "Exact duplicate". The problem is, I can only do that 12 times a day. Between the number of obvious duplicates, and other close reasons, I run out close votes fairly early in the day.
I also see a lot of questions with only one or two close votes, even if they're obvious duplicates. I wonder if that's due to other closers having hit their daily limit?
In any case, I think the limit should be relaxed. Perhaps the limit could increase based on reputation. But 12 a day will only be enough when there are only 12 questions a day that deserve a close vote from me.