This happens occasionally. Two books about the same topic are released in quick succession. And I always feel bad for everyone involved. On the one hand, they are each more likely to enjoy some high profile review space because reviewers can simply bundle them together, but that also means that unless one of the books is quite clearly superior they split the audience. In cases like this, the audience for books about enlightenment-era races to track the transit of Venus is already, I’d assume, pretty small.
But this case is even stranger. The books, both of which came out in the past two weeks I believe, are themselves about a race. I’m sure the irony (note: this is probably not irony but shut up) of the situation helped make each of these authors feel better about the whole worked-for-years-on-this-project-only-to-have-you-steal-my-venus-thunder thing.