Just looking at the migration stats (2k+) I saw 3 duplicate migrations and I just flagged another cross post. That means, of the last 10 migrations, there were 3(or 4) duplicates:
I am thus under the impression, that we at least have a surge of Cross-posted questions, if not a steady increase.
Unfortunately I am not handy enough with SEDE to put together some meaningful data.
Either way: What can we do about cross-posted questions? How can we most effectively prevent cross-posting?