No, we'd only mark a story a [dupe] if it got significant attention in a different thread. I haven't heard back yet, but the most likely explanation is that a moderator (correctly) thought that one Tank Man story on the HN front page was enough.

Edit: oh - I think that one was actually marked a [dupe] by software. I'd need to double check this, but if so, it's because it interpreted the link to the other thread as a signal of dupiness.

Edit 2: yes, that's what happened. When a submission is heavily flagged and there is a single comment pointing to a different HN thread, the software interprets that as a strong signal of dupiness and puts dupe on the submission. It actually works super well most of the time. In this case it backfired because the comment was arguing the opposite.