My steps are getting reported twice:
You will see steps being logged twice, but do not worry! The reporting is the correct behavior because this data is coming from the Activity ring in Apple watch and also from the Cubii Apple Watch app. Both are working separately, displaying twice. However, as the workouts have a unique identifier for each, the data is only counted once.
I tried to put the Cubii app as the priority data input to stop the duplicated reporting.
The Health app on your iPhone will show your total / overall steps, whereas the Activity app will show only those steps that were counted via your watch. Data is not double-counted. When multiple sources are available during the course of any particular activity, the Health app uses only a single data source to update your steps data. That source is selected based on the priority order that you have set (or that has been created automatically - eg by default, Apple devices rank above third party sources).
You may see workouts being logged twice, however, it is actually only being counted once to your overall workout data.