Modifying the Wagon R's 1.0-liter K-Series engine to run on both fuels required replacement of some parts that could not cope with the corrosiveness of LPG. Additionally, the Wagon R Duo is outfitted with an LPG filling nozzle, which is incorporated into the vehicle's main filler cap.
Maruti Suzuki claims that the Wagon R Duo returns a best-in-class fuel economy rating of 33.9 miles per gallon in LPG mode and 44 mpg when burning gasoline, though it's unclear which testing methods Maruti used to calculate those numbers. The Wagon R Duo's release date has not been officially announced. Hat tip to Shrawan!
[Source: Indian Autos Blog, Maruti]