You are definitely not alone.  With DAZ sale season on, I've found to my embarrassment that I've bought many things I do not even remember buying.  I don't know I already have it until I search for it in my order history and find I bought it years ago, or worse, when I download it and find I already have a file with the same name.

I have about 30 runtimes, organized by figure and theme.  (For example, fantasy, nature, horror, contemporary, hair, prehistoric, toon, animals, sports, holidays, materials, etc.)  But these days I don't even bother installing stuff until I actually need it. 

The biggest problem I have is when merchants sell their products at more than one store, or when they move stores.  I sometimes end up buying duplicates then. 

I'm a dinosaur addict, so I keep a draft e-mail at my Gmail account where I list all the Dinoraul products I own.  Since he sells here as well as at Content Paradise and DAZ, I have a hard time keeping track otherwise.