Duo is an evolution of Component, which aims to solve some of the problems that were not addressed in the 1.0 release of Component. To quote Duo's FAQ:
While the release of Component 1.0 solved a lot of the initial gripes with earlier versions of Component, in the end we were after a more radical departure from Component that borrowed some good ideas from Browserify.
component.json should be compatible with Duo's interpretation of component.json, however, this is not guaranteed in the far future. Duo is currently in the very early stages of development and adoption, so, while newcomers might want to consider using Duo, development and support of Component will continue as long as Component continues to be used in deployment situations, and continues to be downloaded as frequently as it has. After the release of Component 1.0, the Component team will begin a roadmap for the next release.