If it makes you feel any better, this sort of happened to me a couple of years ago (not quite, I didn't actually go as far as buying a rifle I already had - but had forgotten about ) when I was looking through my rifles and came across a 1920s BSA Standard and couldn't remember for the life of me when I'd bought it, or why. It was like looking at a completely new-to-me rifle! Up to that point, I knew every airgun I owned intimately, where I got it, from who, how much I paid, where it fitted in the collection etc. I think that was the moment I realised I wasn't to be trusted - with memories at least.