I had a similar, but less extreme, version of that the other day. I was reading an unfamiliar thread from 2010, and I wanted to upvote a comment, but I saw that I'd already upvoted. That was confusing. It happened a couple more times, and I wondered when I had done that. I couldn't remember ever having been to the thread before. Then I saw some comments I wanted to reply to--complete with replies from myself, saying similar things. That was finally enough to jog the memory of the last time I'd read the thread, but it was still a surprise. My opinions, or at least my way of expressing them, had changed a little, but I got the same sense of deja non-vu you did.