If you're uploading directly to Strava and manually uploading to Garmin connect, it is possible you'll end up with "duplicate" rides in Strava.
The reason this will be happening is you also have Garmin Connect set to send rides to Strava. Strava will see the original ride as a virtual ride, and the second one (from Garmin) as an outdoor ride.
How to stop duplicate rides in Strava
If you want to keep Garmin as your central source of all data, turn off the "upload to Strava" feature in FulGaz and just upload to Garmin Connect. Alternatively, go to Garmin Connect and turn off syncing to Strava.
If you try uploading to Strava multiple times in FulGaz, these rides will never show up as duplicates.