Here's a perspective from a Stack Overflow mod, all of this assumes that the question is on topic here:
1) If the question is an exact duplicate by the same user then inform us over at SO, we'll close the question as "Off Topic" as the user is spamming the system (intentionally or not) and we need to shut it down at one end. Because it was asked on a specialized programming site, it's more a matter of getting what you wish for (and keeping the network relatively free from splitting organic search results on that question).
2) If the question is old, but does not have any answers, then I'd flag it for moderator attention and request that it be moved over. Since it didn't have any answers on Stack Overflow, it might get more love here, and produce better content overall, which is the primary goal of the Stack Exchange network.
3) If the question is old, and has an accepted answer, massively upvoted or has answers that are massively upvoted, then I wouldn't bother to request to have it moved unless it's specifically off topic for Stack Overflow (meaning it deals with a specialization dealing with game design and really isn't a language/non-game-specific framework issue). If it's off topic for Stack Overflow, we simply don't want it. But if it's on-topic, we generally don't want to shake things unless it's really worth it.
Remember, if you take really old, highly upvoted content from Stack Overflow, you think it might be great, because you are getting a valuable piece of content. However, when you see it lingering around on the front page for months, and your site becomes associated with those singular questions instead of the nitty-gritty of game development, you'll find those questions can be more of a burden than a boon.
This exact thing happened at Programmers.
That said, in these specific cases, let the question live here, the person obviously is or is interested in getting the perspective from a game development point of view, and it helps to build your community. Remember, we like dupes, as asking questions in different ways helps increase the organic search hits on a topic. If those "dupes" happen to be across sites in the network, then so be it.
Of course, there are always special-cases, but they are exactly that, special, and should be considered on a per-question basis.
In regards to the questions above on Stack Overflow, I'd say some house cleaning is in order (some dupe closing, some not a real question) but all in all, I don't see any reason why these couldn't be moved, but the gamedev mods should speak to the SO mods specifically about that. I just wouldn't expect everything related to game development to be moved over.
And if they are moved over, please make sure that the appropriate questions are closed as duplicates. Remember, it's not necessarily the age of the question that makes it the "non-dupe", it's the one that has the best content.
