I usually know when I have duplicates.  I download my purchases into an archive I've been using for years, and so when I get "already exists, do you want to overwrite?" I know I already have it.

Bundles have always been a problem, even before DAZ screwed up their web site.  But this page is a big help:

http://home.arcor.de/familie.reichow/3D/fwb1.html

You can just search on the item name and see if it's included in any bundles.  If it is, search your order history, and see if you bought that bundle.

Supposedly, the bundle info will be incorporated into DAZ History Manager soon.