Well, it depends on how you architect your system - you can embed whatever you like in your state. I'd probably do something cheap and nasty like embed a modseq counter for each of the different servers into the state string, and if any of them went backwards I'd reject the /changes with a cannotCalculateChanges error.

I mean, you can do other things - but at the point where you're switching between multiple masters in a split brain from your client, all bets are off - so you're going to need to fall back resyncing any mutable data.