Hello @David,
something I tried right now is two upload the identical file twice to Golden Cheetah. First as always the one from GC, then I downloaded the Strava Activity and tried to upload it again. Golden Cheetah did not import the second file and said it is already there, so there is certainly a way to check for duplicates and import files only once. As Golden Cheetah is open source, you might be able to find the relevant code?
Cheers
Boris