Your runs and Apple Health
The same run from two apps
When two apps write the same run to Apple Health, such as your Garmin and the Apple Workout app, Why Not keeps one. Two runs that start within two minutes of each other, or overlap for half of the shorter one, are the same run.
The copy it keeps is the best one: a Why Not recording first, then your watch's app, then any other app, then WHOOP.