Well I have discovered something that I think will help sort this problem out. I noticed that in 'Library' there was an object called 'Hinged Gate 16', and another called 'Hinged Gate USA', and they were both the same size. But for 'Hinged Gate USA', there was also a folder called 'Hinged Gate USA (Do Not Modify)', or something to that effect. This was one case where I could only find one set of the supposed duplicate library parts. So I deleted that folder and 'Hinged Gate USA.gsm', and that got rid of 67 of the 71 duplicates. Two more of those duplicates were easy to sort out, and that just leaves another two where I again can only find one file for each. They are called "DMS_door_threshold' and DMS_door_threshold_OLD', and are found in a folder structure that goes \Library\Objects\GDL Parts\08_Openings\DMS Doors\. So it looks like these too are being duplicated in a 'whole' object, or one that is not divided into parts. And I guess this is what I was getting at - ways that the library manager might find duplicates where there aren't obviously duplicate files.
So can anyone comment on this, or give any advice or resources to help me better understand how this works, so that I can finish cleaning up these libraries? I don't have any experience with objects outside of their typical use in an ArchiCAD project.
And I probably need to point out that I bought Eric Bobrow's 'Master Template', and from that I got a bunch of the US library parts, which I stuck in my 'Library' folder, thinking that they were different to the international library parts, and that they might come in handy. But are they (or some of them) in fact the same as the international library? Because this may account for the large number (now 270 after deleting duplicate texture files) of duplicates between 'ArchiCAD 16 Library' and 'Library'.