First of all, yes, Duetto is similar in nature to EmScripten. Both Duetto and EmScripten are in fact are based on LLVM/Clang.
Details on Duetto are still fairly scarce as is their beta today. Only 100 developers are being granted access to Duetto today, but it's been said that the "C++ compiler for the web" will be properly released in one months time. Duetto is to be released under a dual-licensing model for an open-source project and also an enterprise-backed version.
More details on today's Duetto activity can be found on this blog by one of its developers, Alessandro Pignotti.