No, please don't open a thread if an appropriate thread already exists... It would be duplication.
Necroposting can only be relevant to forums covering timed events, like sports.
The question is what is an "appropriate thread", in my opinion almost every question is unique... otherwise he would not have to ask again.
Also, opening a new thread is always a good idea IMO... after you searched the history of course and didn't find what you were looking for.
We want short threads with a clear topic, and clear answer (if the point of the thread was a question).
That way it is easier to search too.
The WORST examples on this forum are threads like "A thread for little questions" from piratez subforum and similar... I guarantee that vast majority of questions and answers from such threads are LOST forever. You cannot find them, because the forum search only shows the last match per thread. So if you're looking for example for "coelacanth" and that word is on the first page of a giant thread and the last page of that same giant thread, the search only ever shows you the last page... even though the question you were looking for might have been in the first page.
And I could continue naming dozens of other reasons why opening a new thread is a better idea... not kidding.