When a question is cross-posted here and another Stack Exchange site, and it gets good, valuable answers on both sites, is it appropriate to flag one of them for migration so it can then be duped and merged?
Cross-posting to multiple sites is not permitted by network policy. This is to keep people from using an arbitrary shotgun approach in hopes of getting their question seen somewhere. You might contact the user to give them a gentle reminder about the policy, but if the activity continues, you can deal with that user directly.
But for dealing with a rare cross post after-the-fact, if cross-posting hasn't become an on-going problem on the site, I'd be inclined to just leave it be — that assumes, of course, that the post is truly proving valuable content on both sites. I wouldn't blindly remove good content out of spite just to make a statement.
Moderate for content; but don't feel like your hands are tied by the letter of the law in these occasional, common-sense scenarios. I doubt there's a broken window here.