I’m on Android, may be a different situation. But I track with Ambit 3 (sync to Movescount) and Suunto 9/Spartan Ultra (sync to Suunto App). Suunto App syncs to Strava automatically, Movescount to Strava via SyncMyTracks. Suunto App is faster and in the sync log of SyncMyTracks I always receive the message in the attached screenshot and never a double activity.
Screenshot_20190710_072635_com.syncmytracks~2.jpg