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.
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.