Some group instances import fine into Sketchfab, and some lead to extra duplicates floating oddly in the air. I haven't been able to determine what makes the difference. In one model i fixed it by converting the objects to linked duplicates instead, but the other model is quite large and i'd really prefer to use duplicates rather than do that. It would make the file a lot larger, wouldn't it? If there isn't something i've overlooked that can be quickly fixed, i'll convert those groups to single objects and replace the group instances with linked duplicates.
I put an annotation on the floating duplicate groups, which are located close to the origin of the scene. It is note 7. All the objects are where they should be in the scene, the floating ones are extras. The groups called in the blender file 'MIP-Station' and 'IlmeniteReactor' are working fine. The groups called 'Basalt-Fiber-Extruder', 'Basalt-Thread-Station', and 'large lava tank' have those odd duplicates.
I saw in this thread, Blender Upload Issues, that there were issues with group instances before, but the fact that some are working shows it has largely been overcome. I'm hoping i can make some simple change to the problem groups and they will then work.