I took a look at the timeline of the post to see what happened.
It was declined automatically when the review queue vote result ended up as “Leave Open” (by 3 to 1). Your flag recommended it for duplication, but the reviewers disagreed. That ended the recommendation with a “decline.”
Later a user with a gold badge closed it as a duplicate.
Though the outcome seems weird, it’s not really that odd when opinions are divided. Getting to the right end result can sometimes take a few iterations.
(One declined flag isn’t a black mark on your account either, so don’t worry too much about that.)