At this time our apps are not compatible with external fitness tracking devices. We currently support Apple Health, Google Fit and Strava for activity data. Our team in working on upgrades to be more inclusive with fitness device tracking.
Particularly the duplicating data issue is exclusive to Garmin external devices. This issue is related to how Apple Health syncs data from the external device. We do not have scope to control this on our end. Unfortunately we are unable to delete data from participants accounts or regulate how Garmin and Apple Health communicate with one another. We recommend users not use this external device for tracking inside the app until we have integrated this as an approved compatible device