I've come to notice that in SO there are two questions that were closed:
They closed each other saying that the other is the duplicate oO, how can one be opened?
The closed message on What is the best way to parse html in C#? clearly states that it has been closed as Not Constructive, not as a duplicate. Since it has been locked, it cannot be reopened by most users.
It can be unlocked and reopened by a moderator, but this is unlikely as the question isn't within the scope of the site.
The questions have been merged, not only closed as duplicate. There is no way to reopen a question that was merged into another one.
The target of the merge is closed as not constructive and has also been locked by a moderator. There is no way for a regular user to reopen it, you'll have to convice a moderator to do it.
If you think this question should be reopened, make your case here on meta why it should be reopened. But I wouldn't give it a good chance of being reopened. This question has the "historical significance" lock, which means it was a nice question in its time, but it doesn't fit to the rules of the site now.