Top right of the activity page, it will show the source:
Top image was from Dropbox, as Zwift doesn’t forward to Intervals, even though it ends up in Garmin Connect. Garmin won’t forward data from 3rd partys apps.
Bottom image is the Garmin device workout that that stays in Intervals, while the Strava duplicate is deleted. As @app4g has mentioned, some information is written from Strava, eg. name, description ,etc., which is why it shows ~ View on Garmin Connect ~ Strava
MyWhoosh activities would automatically load into my Intervals.icu - a couple weeks ago, without my having to mess around with manually importing fit files.
Now, no more. Not a big deal, as I always run a garmin head unit concurrently - and contrary to the majority of other members on this post - so far Garmin seems to be syncing to Intervals without hiccups