I subscribed to Kindle Unlimited back in December 2018. And I usually purchase books every month. I returned the 10 books I had out and started browsing for more books. In the past, if I clicked on a book that I had already borrowed, it would tell me I had already borrowed that book on such and such a date. Now, I get nothing and find myself about to order a book that I had already ordered. Has anyone else had that issue? Or even know what I'm talking about?
I wasn't aware it ever told you about past borrows - it might tell you if you still had the book checked out. You do get warned on purchases, though. You can see all the books you've borrowed in the past by going to the Your Content and Devices page on the Amazon website (upper right menu) and changing Books to Kindle Unlimited and All to Returned
ETA: if it turns out to be a book you've already borrowed, you could just return that one and check out another. There's no penalty for doing that