There are two cases here:
The question is Stack Overflow specific. In this case the old question should be flagged for an SE employee to migrate here (as it's strictly off topic on Meta SE) and then the new question can be closed as a duplicate.
In the case you mention an employee spotted the question, migrated the original here and then closed the question as a duplicate.
The question is a network issue. While it's correct that it's posted here it does make things a little more awkward. The new question could be migrated to to Meta SE where it would be closed as a duplicate.
However, this seems to be just making work for the Stack Overflow moderators and Stack Exchange employees. In this case it's probably best to either add a comment or, if a little more explanation is required, an answer linking to the Meta Stack Exchange question.