I've seen the behavior on occasion and are a bit curious of the ruling. How can a post be marked as a duplicate of post which wasn't there at the time of the original post? The most recent example I've seen is this:
Why does Stack Exchange require you to click to be turned down?
Posted February 12 and marked as a duplicate of
Why are vote buttons on Stack Exchange shown when they can't be used?
Posted March 14.
I'm told its because the second post had better answers. I'm curious if this is correct. Shouldn't the answers on the later question be merged to the first one, which is the original one?