a PG Music staffer (Andrew) said that they were going to modify the download manager to check and not download files identical to existing ones. I don't know whether that's implemented yet.
Your instincts are correct - there is a huge amount of duplication in each new version, primarily from thousands of RealTracks, space-wise.
Thank you

I will hold out on buying until later in december then, hoping the download-manager will be updated.
Saving 20-30, or perhaps more, gigabytes of traffic is worthwhile I think.
The advantage, though of keeping the old separate from the new, even if there are duplicates, are you then know exactly which files went with which package (so if you go back to install 2014, you would just install the 2014 files).
Indeed

I have the BIAB2014 downloads backed up on external media, and I am not going to delete those just yet.
My hope was that I could put the BIAB2014 downloads on my internal harddrive, and then perhaps avoid having to download anymore than just the new/updated material if I pointed the BIAB2016 download-manager into that BIAB2014 download folder.
The BIAB2014 download-folder is just over 50 gigabytes big. I wouldn't mind if I could skip re-downloading a big deal of that data for BIAB2016 if I basically already have it via BIAB2014

Thanks.