I routinely get duplicate copies of activities showing up in Garmin Connect after I've connected my Garmin unit via USB to my PC and started Garmin Express and allowed it to sync. This problem has been there for several years with three different Garmin cycling GPSs (200, 1000, 1030) and multiple versions of Garmin Express running on two different Windows 10 PCs. Currently running GE 7.4.1 on Windows 10 10.0.19042. Here's the most recent instance from uploading today's ride via GE:
