I had a similar experience. I bought my copy for peanuts in about 1995 and happened to show it to my Dad a few months ago. He said, "I already have that book. You gave it to me for Christmas about 10 years ago." I had completely forgotten that I gave him one. When I told him it was selling for over $1000 (which I thought was true), he promptly sent it back to me. I don't know if he thought I would sell it or what, but there was no amount of convincing that could get him to keep it.
So, I magically awoke with two copies, so that I can lend one out without being such a Scrooge about when I get it back and worrying I will lose it. it was like a surprise present, although I feel (sort of) bad about rescinding the gift to poor old dad.