A duplicate is a duplicate regardless of age.

It's true - more frequently *newer* questions are flagged as dupes of *older* questions, but in this case it wasn't appropriate.

The dupe question for yours is a canonical QA as to why one-click installer stacks (WAMP, XAMPP, etc.) are inappropriate in a professional sysadmin setting. HopelessN00b found your old question and marked it as a dupe of our newer, canonical question on this subject.

You will notice that he didn't just outright close your question - he locked it. The question is indeed off-topic, so closing would have been appropriate, but he (somehow) was able to find a bit of grace in his ([two-sizes-too-small][1]) heart and just lock/dupe your question instead.

To answer your explicit questions in a straightforward manner:

> Why would this be considered a duplicate,

Even though it's not exactly a dupe, for instances like this where 1) the question is borderlin off-topic and 2) we have a canonical QA that covers the topic, we use the duplicate flag as a manner to refer people to the canonical answer instead of just closing the question.

> and why would SF allow this kind of flagging?

Because any mechanism that can help organize and clean up things on the site are helpful. Why should they **not** allow this kind of flagging?

  [1]: http://www.cel-ebration.com/TWO-SIZES-TOO-SMALL.jpg