At least the way it works on iOS, iOS is supposed to keep track of whether you purchased that purchase or not (that type of purchase at least not the 'buy x items' consumables), and require apps to have a 'Restore Purchases' option you can click to load which items have been bought and restore the state of your app.

At least in theory, maybe there's been a few cases where it hasn't worked. I'm not aware of any, though.

I don't know how that works on Android. That might not be supported.