See screenshot below from Garmin Connect - the last two activities uploaded via Garmin Express to Connect both show up as duplicate. This happens at least once or twice a week when uploading activities, and has been happening to me for at least the past three or four years. So on multiple different versions of GE (Windows) and multiple different levels of FW on my 1030.
Doing a search on the forums for this problem shows that it has been happening to quite a number of users for at least the past half dozen years, but Garmin seem incapable of fixing it...