Will iOS 9 reject an iOS 10 backup restoration attempt to the same model phone?
You won't be able to select an iCloud backup that contains data, settings and apps from a newer build. In the past, there wasn't helpful information explaining you need to update the iOS - now generally you get an explanation.

On iTunes with a USB backup (or Apple Configurator), you get similar experience - you don't get to attempt the restore as code to check prevents the operation from starting.

What you can do is sync data using iCloud and possibly computer / cable / third party apps. Most of what you'd want to replicate is easily done without needing to restore a whole backup.