I almost did this. I picked up Shadow Hearts: Covenant as an afterthought when I bought Atelier Iris a couple months ago.
Then, a few days ago, I bid on a copy of SH:C on ebay. Luckily, I got outbid because as I was putting my new copy of Gradius V on the shelf, I "discovered" I already had the game.
So, ever buy a game you didn't know you already had?