When voting to close as duplicate, should I consider closed questions?
Yes, closed questions are valid duplicate targets. I often see off-topic questions that have been asked multiple times before, and closing as a duplicate of an earlier occurrence of the question is arguably the cleanest way to deal with them.
This way you don't have to debate the close reason a second time because it has likely been explained in comments on the original thread. And despite being off-topic, the duplicate target might still contain answers or links that are useful to the asker.