I always find it irritating that questions marked as duplicate show up as closed in the overview. Of course these questions are closed, but they are not closed because they are off-topic or because they have a poor quality. That's why they should be distinguishable from other closed questions by displaying [duplicate].
I'd like to point out the good arguments from Kate Gregory
But when you look at a lower-volume site with a lot of closing (eg programmers.se) the [closed] markers are telling what's welcome here and what's not. When things are closed as dupes they look unwelcome, which they're not. I think the "first impression" is better when dupes are marked as such.
How can I do XY? [closed]
How can I do XY? [duplicate]