Duplicate data in Health app from Garmin Connect data source

Unfortunately Garmin hasn't implemented Apple Health correctly.  It is adding the distance walked/run once when you actually did the workout (which by itself isn't logged per the way Apple Health expects it), and then they log the total again at the end of the day, (which Apple Health does't expect at all).   So, for each day you get:  Every workout distance+(every workout distance+distance covered outside of workouts).  This has the effect of at least doubling the distance (and more if you wear your watch all day).    Even though fixing this would require Garmin to correctly implement Apple Health, I'd encourage you to politely contact support.  Hopefully if that happens enough this might get fixed.  In the meantime, if you want data from your Garmin device to go into Apple Health in a useful way, you will need to use something like RunGap (paid) to transfer the workouts in, and turn off the sync from Garmin Connect.