Plot Coincidences

I just finished Heartstone by CJ Sansom. In it, he uses a plot which featured in my book Shun The Dark, which anyone can download for free from Scribd.com. This is that one of the main characters in his book, Hugh Curtys, is actually Hugh's sister, Emma. In Shun The Dark a missing person has always been female, because the the land tenure required the father to father a son within twelve months - this was a possible clause in the 7th century AD in the Anglo-Saxon world. Since this was a major premise of my book, I could do what some do and claim my idea was stolen. And it was - by me! I nicked it from The Wasp Factory, and blended it with a bit of The Big Sleep. So much for originality!