Starting today (a few hours ago) my phone started giving me two reminders for each tasks, and looking in the app each task suddenly has two reminders planned (both 30 minutes before the task deadline).
The same applies to the web version. Why and when this started, I have no clue. It worked just fine this morning (CET), getting only one reminder (to hand in my time report).
Newly created tasks does not seem to exhibit this behavior.