The "Getting Started" document (https://github.com/redbear/firmware/blob/duo/docs/gettingstarted.md) says
Navigate to the modules folder under firmware (i.e. cd firmware/modules) and type:
make clean all PLATFORM=photon -s program-dfu
For RedBear Duo, the above statement should be like
make clean all PLATFORM=duo -s program-dfu
Isn't it?