Check the devices tab at https://myopenhab.org/. Chances are you’ll find your device listed twice. Try removing one and that probably will fix your duplicate notifications.
As for your other question, I don’t know if there’s anyone still working on the iOS app. Development seemed to drop off suddenly early this year. I wish I had the skills to contribute (and had the free time) as it was a very exciting year of development before that. Disclaimer: not a complaint; just a concern.