I'm about to start on a project which will involve developing prototype native mobile apps (iOS and Android initially) as well as a web-based admin interface and an API for these apps to communicate with. We've got a list of stories already drafted up, however a lot of them are in the format:

As a mobile user I want to be able to view a login screen so that I can sign into the app

If this were targeted for a single platform, I wouldn't see a problem. However, since we're targeting multiple platforms, I'm not sure whether these should now be duplicated eg "As an Android user" or similar. This seems like duplication, but it's work that will need to be completed separately for each platform.

This is the first mobile project we've gone native on - previously it was Phonegap and we lumped all stories in under "As a mobile user". Since essentially this was a web-based app wrapped in native code, this didn't present too much of an issue, but I'm conscious that wholly-native apps are a different ballgame!