It’s not really double, something closer to 1.25x. The bulk of the work is on whatever platform you first implemented the UI on, because that’s where rubber hits the road and you run into things like assumptions the designer made breaking. On the second platform all that’s already done and you just need to replicate.

Besides that, in your typical CRUD app the UI really shouldn’t be the hard part anyway.

Additionally, you’re going to have to deal with per-platform bugs which require you to dig in to the native side to fix even if you go all in on RN or Flutter or whatever the cool thing is this month, and that’s in addition to the bugs with the framework. Finally, major system updates tend to be much more of an ordeal with these frameworks where (for example) mature iOS apps can easily go years without major changes.

There’s also things like https://skip.tools/ which translates your iOS SwiftUI app into native Android Jetpack Compose on the fly.

All in all a lot less trouble than it might seem.