I just checked it again and Strava did exactly this. I just have to make sure that I first save my zwift ride and after that I save my garmin ride. My garmin ride doesn’t sync with Strava then.
Then I got back to intervals and to my surprise it’s the same 
My garmin ride haven’t got synced and I have no duplicate. So, my deactivation and activation was totally unnecessary in the last year or so 
Anyway, my problem seems solved. I just have to make sure that I first save the zwift ride.