It means the moderator decided it was a duplicate question and removed it but the forum software the produces the list of posts hasn't yet noticed the removal and so the entry in the list of posts is still there.
It's something weird about this forum. I've seen similar stuff happen all the time.
For example, when somebody posts a SPAM message, and the message is removed, the list of posts will still show that the most recent post in the thread is from the spammer. I'm guessing that they don't do a dynamic SQL query to get the list of posts each and every time but instead regenerate the list on some sort of schedule and so you have "lag time" like this.
An optimist sees the glass as half full.
A pessimist sees the glass as half empty.
A realist drinks it no matter how much there is.