In two days, I've twice merged questions, because we've had two instances of cross postings (one involved three sites, and is still being dealt with). This has also happened before in many cases, many resulting in merges. The usual other sites involved are Mathematics and Physics.
The typical way that the moderators on HSM deal with this is to contact at least one moderator from the other site(s) in chat, and come to some kind of an agreement on the matter. We've been working off of the idea that cross-posting exact duplicates of questions on HSM and one or more other Stack Exchange sites is not a good thing, primarily because it spreads information out in different places that could fit together better in one.
It would be good to get the community's opinion on the matter, since this sort of official consultation hasn't actually happened. We've been assuming that this kind of thing - not allowing exact cross-site duplicates - is what the community wants. This might not be true, though.
What should be our strategies for dealing with cross-posting? Should we continue with our current plan, or should we do something different?