Owning a champion I didn't buy

I have recently started playing League again after a long, long absence, and things have changed pretty drastically to be sure. Thankfully everything I have acquired from before is still here, faithfully awaiting my triumphant return. So I was happily browsing this new and shiny Collection tab, checking out all the champions that have been released in my absence, and discovered to my great surprise that I apparently own Kai'Sa, whom I most definitely did not buy nor receive as a gift. Can someone tell me what might be happening here? Now she's on sale and I would actually like to buy her, but I can't because she is apparently already owned, and if this free champion turns out to be nothing more than a bug only after the sale has ended, that would be deeply disappointing.