That's because that question has been closed a long, long time ago, when the duplicate notice was added in the question body itself. It needs a ♦ moderator (or a gold tag badge holder, but there are none in the question's tags) to reopen and close the question again; then, the blue notice will be updated and show the correct duplicate.
Reference: Duplicate ...
I think the reason is that some users have the privilege to vote to close questions under certain conditions
... can vote to delete closed questions within 48 hours of being closed,
so long as they score −3 or lower.
Would be great if the help center page be more clear about this.