Genuinely how is this possible? I have nearly a thousand ebooks and I'm certain whether I have or don't have one, because I obtained it deliberately. Are you buying books by the foot or something?
If you can really remember every single one of your nearly a thousand ebooks that you've bought, that's both impressive and baffling to me.
As a hobby, I do a lot of wood working. Recently I was acquiring some books on wood finishes. I accidentally bought one book three times - not all at once but over a several year period. I realized this while trying to organize.
Mainly because they can have very similar / generic titles, and be by different authors. Since I’m using them as a reference, even if I know the author, I can’t always remember if I own this book by them or maybe I own this book with very similar title by a different author.
I did eventually solve my duplicate book problem by making ourselves a searchable list I can access remotely, so now I can just look it up when I'm at the store.
Being deliberate about obtaining them isn't even remotely related to any of that.
I'd imagine this is also more specific to them being physical items, since it's much easier and obvious to look up an ebook if you just look at a list of files wherever you keep them.