Sometimes I come across a book that seems vaguely familiar in concept, but I'm not sure if I've read it already or not. I don't really care to waste my time rereading something that turned out to be forgettable, but I don't want to not read an interest-sounding or recommended book just because I might have once read something similar.
The library doesn't keep records of which books you've checked out in the past - which makes perfect sense from a privacy perspective, not to mention what a huge-ass database that would end up being.
But I've just worked out a way to figure out if you've checked out a particular book before. And the solution is beautifully simple:
Search your email.
If you checked out the book by putting it on hold and having them send it to your local library branch, you'll have an email alert that it's ready to be picked up. If you kept the book until nearly the due date, you'll have an email alert that it's due soon. (Helpful hint for Toronto Public Library patrons: search your email for the call number rather than the book title, since the email notifications used to not contain the title.)
This won't work if you don't use email alerts, or if you delete your emails, or if your primary method of library use is to browse the shelves. But if your library transactions habitually pass through your email, you can find a record of what you've taken out of the library in your email.