Why is it called "Double"?

When testing it's common to say "mock". But mock is actually a specific type of test object. Spy is another. As well as, fake, stub, and dummy. The generic term for a test object is test double. Or, more simply, double. That's where this library gets its name. Underneath, Double uses mocks, spies, and stubs. But instead of forcing you to distinguish between these as some other libraries, it derives it based on your expectations.

So, a bit pedantic, but technically accurate. For a fuller history of these names and good definitions of each, read Martin Fowler's TestDouble.