No, I could understand it if most of them were from a couple of years ago when I started & didnt know anything. I almost still don't. This past summer after my vacation, I installed about 300 pics from my camera. When I ran TWINS (an app that checks for duplicates) it found almost 1500. I have to delete all of them one at a time - really annoying. Almost all of the vacation pics are duplicated & many of the 1500 have the file name but starting with thumb (stands for thumbnail I guess). I don't think I did it.
So you are looking at the Aperture Library with another application and not from inside Aperture? What you have seen using "Twins" probably are the preview images and thumbnails that Aperture creates for efficiency reasons. You should never delete these files with "helper" apps from outside Aperture. You risk corrupting your Aperture library this way, or at least slowing down the Aperture browser, because it will have to recreate these files over and over again. Without thumbnail images Aperture cannot show the images in the browser, and if you delete the previews you cannot share your Aperture library in the Media Browser.
Aperture has a command (Photos > delete Previews) that you should use, if you really do not want Preview images in the library and want to remove the existing previews. And you turn off the generation of new previews from the "Previews" tab in the Aperture preferences.
You cannot turn off the generation of thumbnails, because they are needed.
Thank you so much for responding to my question. Yes, I was using a duplicate finder I got from the App Store called Twins. I wish I had thought of posting to this site before spending so many hours deleting important thumbs one by one via the Finder. I knew I had SOME duplicates but was surprised to find 1500 out of my 8000 pics. I'm not sure why I didn't have thumbnails for all of them - probably because I haven't opened the others for awhile? Anyway, I've got everything backed up & will re-install the library as it was. After that, I'll delete the duplicates that i'm sure are duplicates and not the thumbs.
It's too bad there is no way to do this within Aperture but I can't find any way of finding duplicates there.
Thanks again for your help. I really appreciate it.