One of my books is historical fiction, Daughter of Cleopatra. It's about Cleopatra Selene, a pretty obscure historical figure. I wrote it a few years ago, and looked up what there was on her as a character in novels: the last thing written about her was in the 70s. Seemed pretty safe as a topic, not likely to be trampled on.
Well, that was wrong. A woman named Michelle Moran brought out Cleopatra's Daughter two years ago; there's a book called Cleopatra's Moon coming out right about now (Selene meaning Moon, you see); and in the last year about 5 or 6 fictions of Cleopatra have come out. Which is all kind of ridiculous, since it's been a fallow time for her for a while.
I don't know what started this all. I do know I was ahead of them all (at least from their author's notes and such, I was.) But I missed the magic moment when I could have sold it. So instead, I'll just float out there, as a backup as it were. It seems to be working okay.