duplicates on Activities page, but one of them is empty and can't be deleted

Try reloading the activity list, or clearing the browser cache. It sounds as if you still see the list before you deleted the activity.

thanks, I tried, when I look at the dev console I see that the https://connect.garmin.com/activitylist-service/activities/search/activities?limit=20&start=0 call is still returning both activities (the broken one, and the working one).

Do you see it with the phone app too?

I see a few people in the past who have run into this issue, but they were a while ago and no resolution was listed:
- https://forums.garmin.com/apps-software/mobile-apps-web/f/garmin-connect-web/110794/can-t-delete-activity-activity-not-found/622470#622470
- https://forums.garmin.com/apps-software/mobile-apps-web/f/garmin-connect-web/160923/can-t-delete-activity

yeah, and i see it still counting towards my training status under the "activities list"

And what happens when you try opening or deleting it from the phone app?

the delete just times out / fails on the mobile app. When I try to visit the activity it says it can't be loaded and might have been deleted

Looks like some mess in the database. In your place I'd contact the Support.