Issue when forking a repo that's already forked in an organization user is part of

Case study: I'm part of the [Boltmade org](https://github.com/boltmade), which has [a fork of DefinitelyTyped](https://github.com/Boltmade/DefinitelyTyped). When I ran `github-spellcheck` on DefinitelyTyped, the tool wanted to fork it to `tbroadley/DefinitelyTyped`, but actually didn't because the Boltmade fork existed. Then the clone from `tbroadley/DefinitelyTyped` failed.