In England before the days of eBay and the internet, you couldn't be too choosy on what condition you bought, mostly buying out of catalogues and an occasional mart, if I saw a book I didn't have, I bought it.
I later looked at sale adverts in the Newspapers and sometimes bought whole collections to get a few books if I needed them and the seller wouldn't split them, ended up with multiples of books.
I did this a long time and then eBay came and I started to fill gaps and upgrade lower grade copies.
I guess I just never really got around to selling anything apart from a few titles when I was skint in the 90s
I gave away a lot of duplicates about 10 years ago to gain some much needed space as selling them just wasn't really an option due to time and effort involved, I don't really regret it but a lot of those give away books are a bit pricey now
Yeah, I have been known to have a duplicate or two