unintentional re-read

While reading the book of short stories by J.G. Ballard, I came across a story that I quickly recognized as having read once before. A story called Billenium in which the Earth's population has become a worldwide over-crowding problem. People are forced to live in small cubicles with a roommate, like prisoners in cells.
I could not remember where I had read this story before, so I looked at some of the other J.G. Ballard books I had set aside. Vermillion Sands and The Terminal Beach. These each had a bookmark tucked into the last page, indicating a fully read book. I had forgotten that these too, were short story collections which, it turns out I have probably already read. Billenium was among the stories in one of the two books, along with a couple in the new book that I haven't gotten to yet. Most of the current read is new to me though, and it led to my discovery of two books that I've read, that I was unaware of. I'm only about halfway through.
This has happened before, I special ordered The 57th Franz Kafka by Rudy Rucker from Borders one time. It arrived and I began to read it, when I realized that the first story was one thaat I recognized from another short story collection that I'd read a few years earlier, Gnarl! I compared the two books side by side and only read the four chapters in 57FK that I found didn't appear in the table of contents of Gnarl!.
So I re-read Billenium. It was short and memorable. They discover an unused room but end up sharing it, subdividing it, subletting the space and eventually end up packed in like sardines again just like before. I don't usually re-read books, unintentionally or otherwise. But I have before and might do so again.