I've seen a couple of posts where people cross-posted their questions to another stack exchange, in both cases stats.stackexchange.com . What should we do? Close them if they are verbatim or near-verbatim posts and tolerate them if they are customized enough to be distinct but related questions?
The consensus there seems to be this cross-posting should be discouraged, and that the better approach is to ask on one site, but have the question migrated if it turns out to be a better fit for another. We should probably add this to our FAQ.
As for what we should do ex post when someone cross-posts: my suggestion would be to add a comment explaining that this behaviour is not encouraged (we could write a template for this here: List of comment templates). We could then monitor the situation and close the question here if the cross-posted version seems to be getting more/better answers.