“Have you ever bought a book, started reading it and then realised you have already read it? If so, how far did you get? (Can you tell this happened to me for the first time ever this week!?!) And–did you keep reading??”
I know it has happened more than once with books I borrowed from the library; I suppose it may also have happened with some I bought, but I can’t recall a specific example just now.
If it happens with a library book, whether I continue to read or not depends on how I like it and how much I remember.
I quite often reread books on purpose though – and sometimes buy one that I first borrowed. With a few favourites, I bought them both as printed books and as audio books; or both in English and in Swedish translation.
I suppose the biggest risk for me to end up with two copies of the same book by mistake would be with detective novels by very productive writers like Agatha Christie or Ruth Rendell. I bought some and borrowed others; and some in English and some in Swedish translation. I couldn’t say off-hand which titles I borrowed and which I’ll find in my own bookshelf.